system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is a music conductor necessary?

I know this may be a dumb question, but I'm curious. Obviously the band/orchestra has practiced the song a million times prior to a live performance. Do the musicians still follow the conductor's lead, or do they just play from memory? 

Edit: Wow! I've had no idea this would get such a huge response. Thank you for the answers guys, I've learned a lot of interesting things today!!
gpt
From where you sit in an orchestra, you do not hear every other instrument clearly. This impacts some more than others, but there are people (from experience - the pianist) who can't hear most of the other players - for them the conductor is absolutely essential. Also, a lot of classical repertoire, especially music from the 19th century onward, requires constant speeding up and slowing down, either in the form of tempo changes or as rubato (bending the rhythm for a few notes for dramatic effect) which is hard to do without a conductor.

Also, the conductor is the only person to hear every instrument roughly like an audience member. Thus, they are the only person in an orchestra (assuming a large ensemble) who can hear which sections are too loud and which are too soft, and a large part of the conductor's job is telling individual instruments to change their volume.

That said, there are many smaller ensembles and orchestras, particularly those playing baroque and pre-baroque music, that don't play with conductors.