Why do choirs and orchestras need a conductor, they just look like they're just waving their arms around?

In addition to what's already been mentioned (keeping the beat, signalling when to come in) the conductor decides on the interpretation of the piece. Legato pages are signalled by smooth movements between beats. Staccato or detached pages are signalled by jerky movements between beats. Soft pages are indicated by a decrease in the size of the beat pattern, and loud pages by an increase in the size of the beat pattern. He also signals tempo changes (changes in how fast the piece is played) and other incidental interpretations. In addition he balances the parts in the orchestra, signalling to some to play somewhat louder or softer to bring out different parts of the music.