Because that's the convention in lisp?

Technically, it's not the convention in Common Lisp, but rather Scheme and Dylan. But yes.

Both Lisp and Ruby all follow that convention. Probably other languages, too, but I'm too lazy to look... :)

In Ruby, it does _not_ mean mutation. It means 'danger.' Sometimes that's mutation, sometimes its other things, like throwing an exception rather than returning nil on error.