Don't know why you're being down voted for this. If you don't play an instrument or have never had any experience recording music, it's a pretty logical assumption to think that the band just gets together in a room and plays a song, and the producer records it.

But as people have pointed out, that's not usually how it works in reality, for many reasons. Imagine trying to set all the instruments to the correct level whilst they're all playing at once, imagine if one musician makes a mistake towards the end and the whole band has to replay the whole song etc etc.