Guys, lots of people above saying it will damage the phone etc... or hot battery. Please read the other comments.
IT DOES NOT ACTUALLY USE YOUR PHONE TO MINE. It's all simulated, they did a big premine of ETN which will be handed out to simulated mobile miners. It's basically an airdrop that introduces people to the world of cryptocurrency.
I think that the average phone should be given around 10 ETN a month, which isn't substantial for people in first world countries, but imagine if you're in India, Africa or the Philippines? 1$ to them is a decent amount of money, some of these people get paid less than 100$ a month. So this will be hugely beneficial for them. It also gets more people involved in crypto which is good for everyone.