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:

-Amp?
-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.