Up until now, there have been only two ways to earn loot boxes in Overwatch: Earn them and Buy them. Now a new website called GammaNow is allowing you to take the bitcoin route and offer a third option: Mine them. To be more specific, your computer would be doing all the work in order to obtain these through the website, you'd basically be doing nothing beyond allowing your PC to do the work necessary. Much like mining for bitcoins where you allow your PC to utilize unused processor server power to do minuscule work to slowly rack up credit toward bitcoins, GammaNow uses your idle PC to do work on their network to slowly rack up credits for you. You then turn those credits in to purchase items like Hearthstone or Overwatch loot boxes or Riot Points with League Of Legends.
There's absolutely nothing illegal about the situation, it's just a matter of whether or not you want to allow a third-party to use your PC while it sits idle. If you care to get more in-depth about how the system works, Forbes has a much more intense article on how it works compared to other systems that utilize the same algorithms. Just remember kids: Free isn't always free.