We all have in our pockets a basic MP3 player- our phones. So you could most definitely use your cellphone as your MP3 player. However, there are benefits in choosing a designated MP3 player for your running or exercise.  Typically if you find MP3 players designed for this, they will be lightweight and smaller, which is always handy when you're out exercising. They also tend to be more durable than cell phones, so if you're someone who's worried about dropping your expensive smartphone, you might want to look at investing in an MP3 player.