An optional boss is a boss fight where you don't really need to encounter it - and in some cases may never encounter it if you don't know it's there - in order to beat the game. You can get from start to finish, see the true ending of the game, without ever needing to fight these bosses.
And sometimes that's for the best.
While most optional bosses will give you lots of goodies, whether they be experience points or equipment, some don't feel the need to give you either. Or if they do, what they give you is insufficient compared to the hoops you had to jump through in order to get to this point.
Optional bosses can be lots of fun and great challenges for the player. But these are not those cases. These are the super hard optional challenges that either aren't worth the preparation needed to get there, or have some kind of stipulation holding them back from actually being worth it.
Everyone loves a good challenge, but only so long as they get something out of the bargain. These bosses don't give you that.