Slitherine

This may sound like a silly question, but what exactly does this do?

Before you your turn, you see what the other play did in their turn, i.e. what units they moved, where they attacked, etc. Of course only those that are not hidden by the fog of war.

Okay, thanks. I'm not sure why one would want to de-select this option then, unless patience or time was in very short supply.

You won't believe what some people like to turn off ... personal preferences are very personal at times. Offering an option is never bad imho no matter how obscure it may be. It doesn't hurt anyone as long the preset is sensible and if anyone prefer the obscure way to play so he can do. There are some GUI experts claiming offering to many options is bad as it confuses users but i disagree personally. Make a sensible preset and hide the more obscure options in an own tab marked as "advanced" or "expert" and whoever goes there does it at own risk but he can fine tune the behavior of the program to his own liking.

After I heard that some players on conventions while being watched activate "full brightness" in Left for Dead I believe, since otherwise they "don't see anything in the dark", I won't question anymore what options people will activate

And the ones who turn it off usually uses massive amounts of paratroopers and recons because it's very hard to spot them when it's turned off.