What is the benefit of a good transport?

I'm trying to understand the role of the transport - i.e., what does it need to do and what differentiates one from another? How old is too old when it comes to used transports? I know it reads the disc, but I don't really get how one is different than another. Thanks for any advice.
It's really just zeroes and ones, right? Better transports provide more stable platforms and better read capability. A very knowledgable friend with an EE degree who competently builds his own tube gear was not a believer of this until I sold him my modified CEC 2000 transport which is the most analogue he's ever heard. He was shocked at the difference. Much smoother and detailed than anything he had owned, so it is real.
I don't know exactly why transports sound different, but they do! Alright, alright, it's about 0's and 1's but... I think it's about jitter, don't you think? So in theory the transport with the less jitter is the superior one, I suppose... Have you heard a good sounding jitter troubled transport? In that case, just forget my preassumptions, I'm not a digital guru.

Jitter is the biggie. In non technical terms, it is 0's and 1's, but if the timing of their arrival to the DAC is off or not even or smooth, etc, then the DAC is going to have to work extra hard to turn those messy numbers into music. And down goes the sound quality.
So if it is about jitter is that something that new technology has improved upon or is an old (say early 90s), high quality transport more or less as good as any newer transport?
It is my understanding that jitter can be dealt with at the transport level, the DAC level or both for that matter. A signal is passed, along with the ones and zeros, to the DAC and if the DAC is designed to reclock the signal, jitter is dealt with.
As a non digital expert (not an engineer), how would a DAC know that the signal it receives needs to be re-clocked? I can imagine a transport doing that, since it is reading the source material.
I am no expert either, so those with more knowledge than I feel free to correct me.

The DAC doesn't "know" the signal needs reclocked, if a DAC is designed to reclock it will do this every time. As noted before, the "clocking" information is sent via whatever connection you use, whether a DAC reclocks or not this is passed along. Also, as I am led to believe, doing it at the DAC level is a very good place to do so, addressing it as late as possible, just before the D/A conversion.
So if the DAC really solves the problems, then all you would need is a $30 DVD player from Walmart as a transport.
:) Now we are getting into areas where those with more knowledge will have to chime in; I suspect there may be a little more to the reading (transport) side of things, but how much, great question, hence the reason of the post. Also, the DAC must reclock the signal, not all do.
Jitter is essentially the inherent problem of a digital signal (bits of information) living in an analogue environment (time). Errors arise because the transport must read the information off the disk and convert it into a time encoded digital stream in realtime (hence the term "stream"). A computer copying a file will read over a sector of a disk as many times as necessary to get the information properly. An audio disk gets one pass. If it misses information or gets it wrong, it has to rely on imprecise algorithmic information further down on the disk to correct the information - this is known as Error Correction Codes (or ECC).

This task is difficult enough under ideal circumstances, but have you ever taken a good look at your CDs? While the information encoded on them may start out correct and pure, errors in the pressing process, degradation of the disk, material imperfections, scratches, warps and so forth make corrupt the data further or make it more difficult to get at. This is why things like Mobile Fidelity Labs gold disks sound better than a standard disk - better materials less prone to degradation. There are other factors that come into play such as the amount of vibrations caused by the motor spinning the disk, other equipment, or from music itself causing errors. Electrical interference from a cheap power source or interference from other equipment can cause read errors as well. Lastly, the quality of the electronics that generate and send the signal down the road to the DAC also play a part in the quality of the signal.

Again, the basic problem is that the CD is using a digital medium (bits of data) in an analogue application (streaming). This is one of the reasons there is so much excitement about file based audio (Sonos, Slim Devices, PC Audio, Mac Mini, etc). Pull the file off the disk in a digital application (file ripping) and get a digital source that can produce a more accurate stream from the beginning and you have won that part of the battle. Assuming your software is set up properly, the problem then is reduced to transport jitter to the DAC (which can still be considerable and problematic). For many people, they simply prefer to spend a good hunk of money on a really good CD player with quality components all around (transport, clock, DAC) - the shorter the signal path, the fewer components to go through, the less chance for error and corruption.

So yes, it is all 1's and 0's. But unlike applications for your computer which 'read until right,' a music transport gets one opportunity to get it right. Given the volume of data, that is a lot to ask of even the best components. The better your downstream equipment (good DAC, revealing speakers, etc) the more you will notice problems of the source.

Does that help?
Thanks everyone for your responses, especially Shazam. I'm still wondering if it's a technology where new may be better or has it peaked and new doesn't really mean better. I currently use the toslink connection from my mac to my krell studio 2 dac playing lossless files and it sounds really good, but I still think it sounds better when I use my kav300cd - I don't know if it's the cd player's dac or if it's the transport, and the cd player is out for repair so I can't test it yet by using the digital out to the external dac - I do know my sony za20es used as a transport to the krell dac doesn't sound as good as the all-in-one kav300cd.
Yes, thanks Shazam for a description of how things work that a layperson can understand. I didn't really think much about transports until I purchased an Esoteric X-03SE with the VRDS-NEO transport. Aside from being a wonderful sounding machine, the confidence that the transport inspires is revelatory. When I load a disc and hit play I simply KNOW I am getting the best read of the data because of how the mechanicals look, feel, sound. The transport is really an engineering marvel.