Well, for example:
addslashes()... Returns a string with backslashes before characters that need to be quoted in database queries etc.
stripslashes()... The opposite
strip_tags()... This function tries to strip all HTML and PHP tags from the given string.
There are some nice date/time-formatting functions too.
All in all that's nothing that couldn't be done with good old regexps, but I don't always want to fuzz around with regexps.