When does someone need a DAC

Always looking for better sound, but I have a higher end CD player. What is a DAC going to do for me... I thought every CD player has a DAC built in?
well... never. But a separate DAC is useful sometimes.

I own several CD machines.. And only my Sony SCD777ES is better using it's own internal DAC for CD than my separate DAC. and using the CD machine as transport only feeding a DAC.

With a higher end CD player.. adding a DAC may, or may not do anything at all for your quality of playback. And the DAc may not even be as good as no separate DAC.

I like to use five disc CD machines all the time. And THEY do benefit from using a standalone DAC.
I currently have FOUR CD changers connected to my DAC.
Well, for one thing, you can then use a computer as a "transport" and play digital files.
Yes. A CD player player does have its own internal DAC. It has to, otherwise it wouldn't be able to play the CD. I think a good place to start would be listing what kind of equipment you currently have. If you already have a high end CD player, its very likely you won't need to do anything.
When it walks like a DAC and quacks like a DAC
Digital cables from various sources into a decent DAC and then onto the pre or integrated. I used an older CD player's DAC which had digital in, now I use an Oppo's DAC the same way. Thing to remember is not all DAC are created equal. The older, the less expensive, etc., equipment could use an upgraded DAC.
Bobmclean, Good DAC when used with your existing CDP might give you better sound. On the other hand when you connect DAC to computer you will become independent of transport (hard disks are cheap) with ease of use, being able to find music quickly and create playlists. In addition you will be able to protect yourself from disasters, like theft or fire, by creating backup HD and storing it in safe place outside of your home (I keep mine at work). Also, you will be able to bypass limitation of CD (16bit 44.1kHz).