Noob question about big crystals.

That’s literally all that they’re used for

Basically yes. They can be useful if you don't want to level grind

Yep. B4 it wasnt worth it to use them since leveling up in the training tower provided easier sp.

But now that we have special training maps using them is an option. Esp for units (armors) that u can trow in by themselves and have the AI do all the work for you.

Yep. I really only use them for f2p units who I just need for a guide or something, or if a unit is just off from lvl. 40 and I don't feel like going into the training maps again.

It's helpful to have them on hand in case you need to quick-level someone. Like if you're following a guide and it uses a level 40 4-star unit that you don't like so you haven't bothered leveling it even though you have it at level 1. Also handy for leveling healers, since they can be a pain, but then you still end up needing to grind SP on them.