The Utopia Boot Disc is just like it sounds. If you have a game that isn't selfbooting (it won't load and play like a real game), then you first put in the Utopia disc, wait for it to load, then open the DC, take out Utopia, put in the game, and wait. It'll load it for you.
The games with 2 sessions just put the booting process and the game on one disc, but, you have to have enough room on an 80 minute CD to have the boot process on there. Some games just cannot be ripped small enough to allow room for both the game and the boot process on the same CD.
At this point, if a game hasn't been selfbooted already, there's a reason why...it's just too big.
EDIT: There are a few nonboot games out there. I'd say over 90% of the DC ISOs selfboot though.