how come in saltwater tanks you need to have a power head/wave maker?

Water movement is the key to success in a saltwater tank.

How you move the water is up to you. You can use wavemakers, a bunch of powerheads, an oversized filter, add on a skimmer, etc. The rule of thumb is to have 15-30+ times the volume of the tank in water movement an hour. Short answer as to why: it helps stabilize water chemistry and avoid dead spots.

Depends on other factors, but yes, some salt tanks require extra circulation and wave makers produce varied currents which can be beneficial, especially in tanks with corals or stuffed with live rock.

Note: being anonymous makes it harder to keep track of your previous questions to help give correct advice aron.