It does not mine anything as of now. Thus no battery drain. This is because the pi ecosystem need a certain number of users to actually start functioning. So until we reach that threshold and people start mining (as a node on PC) the coin practically has 0 value
If you're a techie and want to know how pi actually works, refer it's White paper it's on the Pi Network discord or the app or the site basically it's everywhere
I agree that there is no real value but we need to think about the marketing. A lot of people know about Pi network ant that has some value to it. If they can do it right maybe there can be something from it.
There is no way a mining can be done in the phone device, it's absolutely impossible, the hardwares used have greater GPU space and with good efficiency compared to the ones in phone device even in PC, Pi network teams are sort of scammers trying harder to get access to peoples storage files and get rid of all their secret keys, who knows if the recent hack of wallet is operated by pi network hired scammers. I may be wrong anyway.