What does rotating the compass do?

I've unlocked field E3, it has a little compass on it, if I click on the compass, I can change the direction it's pointing at, after rotating it a full circle I got an achievement. Bombing it didn't seem to do anything at all.

What use does this compass have? I'm assuming it has some effect in the game, especially since it also shown on the world map that the level has a compass. What do I have to do to get it to do something?

Yes, the compasses do something.

When you set the compasses correctly

the reward is

five fields, when beaten, will each yield 1 talisman fragment of set rarity

after that, the compasses have no effect.

There are 7 compasses in the game. Each must be set to a specific direction (random by game, so yours will be different from mine.)

On the load screen you will see 7 gems... if they are level 5 gems they are correct, otherwise they are wrong. Each gem corresponds to a different compass.

It's worth mentioning you do NOT have to beat (or even play) the level to change the compass setting, merely load the level, change the compass and then reload the level to see if you got it correct.