Why do we need Functions?

[PhilOfPerth]

I read that there is only one difference between Subs and Functions: a function returns a value, while a Sub doesn't. But as far as I see it, you can use Subs everywhere that you could use a Function. If I call a Sub, with variable parameters, I can work on those variables and (as long as they're Common Shared) I get the changes back in the main prog. Is there some other subtle difference? if not, it seems like Functions are an unnecessary item.