Some people have poor eyesight. I was always fussed at on Star wars Galaxies for using it, because my friend had an eye disease (we would hunt ship parts in space and reverse-engineer them once on ground again, so we were around each other a lot). They never complained in the PVP zones, however
Some serials in games are easier using caps lock, also, when searching for duped items, ie. cheating players.
It's the only key in probably 100 that people bitch about. This says a lot.
It's used heavily in writing COBOL and FORTRAN.
It can be re-mapped to other keys like CNTRL. Some people map the Windows Key here as they don't have one on their keyboard.
It's used for typing may constants.
Law Enforcement uses it, as well as its use in many legal documents, especially in the names involved in the documents. One reason is so that the name is completely and irrevocably understood.
It's used for typing Headlines.
It's useful for accented letters, like like À É È Ç ..
There are many uses a normal everyday person won't comprehend. Why? Because we're not in the position to understand. And, because we don't understand, that doesn't mean it's true that it's not useful and even necessary. Maybe we should each say 'it's not necessary for me'.