BeerBorg

Simple buckets are not always airtight. Without the pressure from an active fermentation O2 can seep through the plastic used to make buckets. Plus you want to minimize the amount of surface area exposed to air. Secondarys are not required. I do mainly to get less trub in my cornys. With some of the Belgium style beers that I brew need that extra time bulk conditioning. I have pretty much always used secondarys. A habit picked from making wine.

I dont always use a secondary. The four reaons I usually will are:
1) Lots of dry hopping (I like hops!).
2) Fruit additions.
3) Extended conditioning, which usually always leads to #4.
4) I need to free up a fermenter cuz I'm Jonesing to make some beer (the "just because" reason).