For the Maps!!!!!!
That and for the level of detail. Sure I can make my own game world, but sometimes it is nice to just set back and let someone else drive.
As to the "Mis-mash", I don't mind. I do like that they use real world history as a base, so that if I want to flesh it out, I just need to make a trip to the library.
The realms are great for "one nighter" adventures, just pick what kind you want to do; Dungeon, City, Ruins, Wilderness, or High Sea, somewhere in the realms, there is an adventure for you.
Many of the Novels are "pulp novel" quality, but even the bad ones give you an "insider" view of a section of the realms.
The only problems I have are:
The inconsitancy of Psionics in the realms. The rule books say no psionics, but the novels are full of psionic characters.
The old FR series came out too fast, but now it come out too slow to feed my addiction (Need more cheeeeese!!)
And the realms purist can be anoying. My personal realms have a Half-evlen nation on the Dragon Coast, Nimbral is an Island of Psionics and I adapted the Spintered Mind from Dragon #281 into a psionic branch of the Harpers.
I posted some of that once and got a lecture on how wrong I was.