I like using the biscuits to make my pets bigger when they have an especially cute name (which is, like, all of them) or on the pets I know someone, usually a guildmate, has been unsuccessful in farming.

Haevlyn, m'dear, your [pet]Jademist Dancer[/pet] wants to work that pole all the way to 25. You'll like her moves.

Look at that poor Tideskipper. Poor little guy is practically begging to be all grown up.