All of the amplification work is done by the three tiny chips at the bottom. Yes, it really is free. No, they won't spam you.
Headphones are marvellous things.
A good pair has sound reproduction qualities that would cost ten times as much from loudspeakers, and they may not get the floor moving, but they make up for it by letting you listen to whatever turns your crank at jetpipe reheat volume without inducing neighbours to call the National Guard. If more than one person wants to listen, headphones are a problem. You can put a splitter on a headphone jack and plug in two sets, but then you get half volume. Furthermore, even with only one pair of headphones, the power output from the headphone jack of many portable devices isn't sufficient to cut through the background noise with which they usually have to compete. The oddly named Boostaroo is a low cost solution to both of these problems.