The most obvious Reason Is, That it is a great leveling spec. Your pet does more damage and holds agrow a lot better and then there are the special pets that only a Beast Master Hunter can tame.

The following is a list of the pet’s that only a BM hunter can tame.
Devil Saurs
Core Hounds
Shale Spiders

To tame any one of these pets you must spend at least 31 points in the beast master tree and get the last talent Beast Mastery this allow’s you to tame these very Exotic animals and add them t your own stable. Each one has there own ability and talents, So why not go out and enjoy your very own special pet. Not every one will have some of these as some of them are extremely rare spawns.
These are the really fun pet’s so have a blast with them I myself love my Devil saur Blind Spot. he’s big and you can’t hardly see around him. So enjoy hunters.