The days of mobile recordings being limited to grainy video and maybe-understandable audio are over. High quality cameras in cell phones have become a huge marketing point for cell phones, but unfortunately we haven’t seen nearly as much improvement in the on-board microphones in these devices. Whether you’re using an Android phone, an iPhone, or even a tablet, using an external microphone can drastically improve the quality of your recordings. You may not want to try and record vocals for a studio album through your iPad, but recording the audio of an interview for later publication is much more within reach. Take a look at what external mics there are and what it will take to get them working with your device. The benefit they offer far exceeds their cost.
You can read more about USB and other mobile mics here: http://audioundone.com/microphones-and-mobile-devices