I am no more likely to type "sl" instead of "ls" than I am to say "mu" instead of "um." Getting the current directory listing is often the first thing I do when I switch back to a command prompt, even if I don't need it. It's an involuntary reflex.
I am used to typing "ll", so I get very frustrated when ll isn't aliased. Also, this means that I have probably never typed sl since I rarely type ls.
I don't think I've ever misspelled ls as sl (though I've done ls- alf and derivations plenty of times). Now that I've installed this, though, I intend to do it often.
The man page has a much better description:
sl is a highly developed animation program, which corrects your miss typing.
I am more likely to type ;l or la by accident but la is an alias for ls -a so that sort of works :)