What do the skulls in Halo 2 do?

Question

Well I know what they do and how to get them by looking and videos and walkthroughs..

But can you use the effects later on in other levels? Or only from that point on in that level?

Because some of the skulls have some weird effects that wouldn't help you at all during that level.

Answer 1

Random effects.  They're just easter eggs, and i believe that there's an achievement in finding them all.

Answer 2

they make the game harder