Pre-Amp vs. Integrated...why?

I have been a vinyl listener now for over a year and one concept that escapes me is the "need" for a vinyl pre-amp.

I understand the need if your current amp does not have a Phono input, but if you have an amp that has a phono input, why buy a phono pre-amp to run your TT through?

Is it standard thinking that the integrated Phono input is always substandard to a good phono pre + amp setup?

Are there lines that are drawn so to speak, maybe from a dollar standpoint, that clearly mark the quality in phono stage? (i.e., are cheap $50 pre-amps worse than a $300-$400 integrated or vice versa is a $400 integrated usually going to get crushed by the $500 pre + Amp setup?)

Are there exceptions where a lower cost integrated can hold it's own with Pre and Amp setup?

I am in the process of assembling a second Vinyl listening station in my house and want to start from scratch with new and different equipment. Something different from what I have had over the past year.

Can anybody clarify this or demystify this for me?

My current setup:

-LP120 turntable / AT440MLb cart
-Marantz 2220B Amp
-Infinity RS2000 bookshelf speakers

My future/second setup:

-TT? (I have ideas)
-Elac B6 Debut bookshelf speakers (already have)

Room specs are about 12x12 and this is for vinyl listening only. Other stuff is a bonus but not necessary.
Just to clarify you understand what the phono preamp is doing right?  its main job is to decode the RIAA curve and to amplify the very small signal from the cart so your preamp can use it. (preamp regardless if integrated or not). The decoding is really the main job with out that you can't play a record. then your into what gain and loading you need for the type of cart you have.
J, the above explanation nails it for cartridge decoding. Like you I spin vinyl for now and really enjoy the combination of Project’s Tube box 2 with a Primaluna Dialogue HP with 8  kt150s driving ML Ethos speakers.
Have fun with your search!
The Parasound JC3+ is a great phono preamp I use it with my VPI prime turntable, I started with a Schiit mani phono pre but the JC3= just blows it away, it's not cheap $2,950.00 but it works great and it is for both MM and MC cartridges.The Schiit mani is only $129.00 but it's good to start out with.
Got a lounge phono preamp in to a Sansui tube receiver. I did this because the phono pre on the sui was garbage. It now sounds great. The lounge cost me 300$. 
There are many reasons for using a preamp and amplifier(s) versus an integrated amplifier or receiver.

In my case, my speakers (Maggie 3.6) need significant power, more than available with a receiver.  I am running Mac MC501 mono blocks that make them sing.

I am looking into getting back into vinyl and my tube preamp accomodates that if I make that decision.

Sometimes other components make the choice for you.