Technological standards are a marvelous thing that greatly help consumers. You know how nearly all smartphones these days have a 3.5mm headset jack, HDMI-out, and MicroUSB port? These are standards that many companies agree upon to make the lives of consumers easier by reducing the fragmentation of devices, accessories, peripherals, so on and so forth. The MHL consortium promotes the MHL standard that aims to help keep your battery charged.
When you connect your phone to a television for HDMI use, your battery will drain. Oftentimes it will drain a LOT since beaming this information is very media intensive. But your phone is plugged into a huge electronic device that is plugged into a wall, so why should you have to waste battery? With MHL you WON'T.
If both your television and mobile phone have MHL, your phone will actually charge while connected to a TV via HDMI. If you do not use the HDMI-out feature of your phone very often, When you does, you will not worry about your phone dying.
The MHL standard was initiated by 5 companies: Samsung, Sony, Toshiba, Nokia, and Silicon Image.