There are also headphones with built in players, I don't know how the quality of those is though.
Because you might want to stream audio from the Internet, or you don't want to waste time converting newer audio formats to MP3 and then copying them over, or you'd rather not have cables around you when you work out, or you might want to track what you've listen (particularly useful for audiobooks and long podcasts), etc.
Yes, these are not indispensable life necessities nor anything like that, but given the choice, I'd rather beam the audio from my tablet than carry an extra player.
Sony still does players but they're not very good.