What does the compass do?

[FettPrime]

It's pretty expensive so I probably won't feel confident splurging on it without knowing the effects.

[gibmepepper]

It points in the general direction of the crystal inside the dungeon. (If it has one)

[Ninjaboi8175]

Like gibmepepper said it'll just point you to the next crystal's general location. It's useful for when you have multiple paths you can take in a dungeon but want to get to the crystal fast without exploring everything. The longer you're in a dungeon the more things spawn so it can be a good way to secure crystals earlier so that you can loot until enemies start showing up and you can dip out.