All of reasons, what already were meaned, and few more.
1. lvl pets, what I need fore some achievements, like 75 pets on 25 lvl before or big adventure in this patch.
2. lvl pets, what will be usefull and have unique abilities, some time few of them.
3. lvl pets by families, from smaler to bigger.
4. lvl pets step by step with small goals - all pets higher than 10 lvl, higher than 16 lvl, etc.
Actualy, all this reasons runs about same pets. Most unique, cool and usefull pets are from small families, like mech or ud, in time like 100 from 150 beasts or critters are crap, what is lvled to some point by 1 or 2 battles against pet masters.