For a wc3 map, I think there's no point...
but for stand-alone games for example, there is a point... 1 is that there are lots of scripts that aren't free to use, so if you don't have money then you need to write it yourself...
and another thing would be (this is a personal opinion, for both wc3 and others) is well, practice and experience... trying to make your own script even if somebody already made that can be quite a good way of practicing... after all, once you're done you can then compare your resource with that of the existing one...