Oh yes, I have a question-does a maestro actually do anything?
EF's answer is great, but I'll expand on it...
Absolutely. Does a sports team need a coach? Why can't the players just go out and, well, play? For the same reason that an orchestra needs a conductor.
The conductor is the leader. Just like everything else, when a group of people are trying to accomplish a shared goal, it helps if someone is the leader. The conductor guides the performers through the piece of music. The individual musicians all have tremendous talent, but someone has to control that talent and guide it to produce the orchestral work. The conductor also plays the roll of an "interpreter" of the piece of music. Interpreting what the composer desired. And that's were a lot of a conductor's genius comes into play. Different conductors will run an orchestra through the same music in entirely different ways; controlling the tempo (speed), volume, and emphasis on different instruments through different movements.
And on a less esoteric level, you also have to understand that sometimes as an individual musician in an large orchestra, it can be difficult to hear the "big picture". The conductor, out in front of all of the instruments, can hear the combined output of the entire group. (S)He will hear that the oboe's are a bit too loud, or that the 2nd violins are a little too quiet, or that the timpani is just a little bit ahead of the beat; and (s)he'll issue commands (via that arm-waving) to adjust individual sections (or individuals) accordingly.
