The yes command will repeat whatever string you give it, so it's useful for load/traffic testing. Speed helps this particular use-case.

That's a possible secondary use, but not really what the command is for.

Why not just use /dev/zero or /dev/urandom for generating load?

Because the yes command will repeat whatever string you give it. If you need to parse the incoming load in a certain way, /dev/{zero,urandom} won't be suitable.

yes will repeat arbitrary data (including binary).