A big part of the mp3 player for me is that I detest much of the music played in gyms. Most top 40, pop, r&b, I just don't like it. I have to listen to that **** all day at work and then in the gym too--no way.
I must admit that I've forgotten my mp3 player on many occasions and I did just fine without it. I also can hardly remember what songs I listened to today. But, it enhances the atmosphere I desire to lift in, helps me tune out all the bull**** around me, and simply makes it easier for me to reach the desired level of intensity for moving maximum weights.