For me to keep track of things I want, things I'll probably never have, and things I can never have enough of.
Go-getters: all manner of tutors and ramp-spells.
Mana-makers: lands, rocks, and dorks.
Tokens: any and all things that make, or have to do with tokens.
Counters: go figure ;)
Tribal: lords and the like.
Deck specifics: things Inneed for existing or upcoming decks.