Hmm.. interesting.. I've never given it any thought.
You can activate a script with simply adding the exec command to the on activation field.. So there must be some other reason for it.. ummh..
Maybe it's a way to execute a script with automaticly passing the parameters group, position and target.
So in a script you can have
group1 = _this select 0
and it will select the first parameter passed, being the group. In a way it would look something like
[groupname, getpos wp1, targetname] exec "script.sqs"
I'm only speculating.. plus it can't be very important sense none of us has ever heard of it.