Why would you need a slash at the end of a URL?

The browser will treat them differently when it comes to relative URLs. A page at:

http://server/path

with a relative link like:

<a href='other'>

will resolve that link to:

http://server/other

replacing path with other. Whereas if the starting URL was:

http://server/path/

then the resolved link would be:

http://server/path/other

If path is a directory rather than a file, most web servers will automatically redirect from:

http://server/path

to:

http://server/path/

because that's almost certainly what you meant.