Makes it clear you're naming a directory and not a file.

I also alias 'ls' to 'ls -F' so that directories have a / appended, makes it easier to understand the output.

For me it's because of rsync.

It's handy when the directory might not exist, happens all the time in git checkouts. Raise your hand if you've ever moved something to tmp and created a file called tmp.

Usually it's tab-complete adding the slash though, I don't go typing it in.