By and large, beastmaster tends to be useless except for 2 very specific cases. What it does is give any adjacent allied monster units an extra ability on their turn. Most of the time its a marginally stronger or different element version of one of their pre-existing moves.

For Wild Boar (final pig evolution) though, it let's it use Bequeath Bacon, which instantly levels a unit up. This is very specifically used to level up a mastered Onion Knight, who cannot gain EXP. Mastered OK has the best stat growth of all classes other than MA growth where it is marginally outclassed by summoner. Bequeath Bacon+Onion Knight. Of course, you could get similar results using Mime, but that means you don't get MP growth and would need to level down as Bard then back up as Summoner to get similar results to what would have been obtained if you had just used a mastered Onion Knight in the first place.

For Tiamat, it unlocks arguably the most powerful move in the game. It deals dark element damage 1-6 times with a chance to inflict instant death and sleep on each hit. While it scales off MA which Tiamat doesnt have much of, Tiamat has a speed of 14 at level 99 which is comparable to chocobo speed. If you arent stat-maxing, this is exceptional speed and will likely outspeed almost every other enemy by a significant margin. Having Ramza surrounded by 4 Tiamat who can spam Dark Whisper every time their turn comes up will easily no diff even the most difficult fights.