https://www.gamespot.com/articles/xbox-boss-explains-why-backwards-compatibility-is-/1100-6453776/

One nice feature on Xbox One is backwards compatibility support for Xbox 360 games (and original Xbox games soon). If you own older titles on-disc or digitally, you can play a growing list of these on Xbox One at no extra cost, which is a pretty nice option to have. Even if you don't use it all that often, it's nice to know it's there.

Xbox boss Phil Spencer told Wired recently that one reason that backwards compatibility is so important to Microsoft is to allow for the preservation of games for new audiences. It's an enticing and potentially lucrative business opportunity for publishers to be able to sell older titles to new players but that's just one element of it.