The ability to make a popular game franchise exclusive to their gaming hardware. Just take some money you would have spent on advertising and dump it into buying some franchises for exclusivity.
Also since Mojang is a European company MS can no doubt use some of it's $90+ billion stockpile of money outside America without getting hit for taxes.
But minecraft already released on ps4. Unlesss MS buys and takes back the minecraft of ps4, there is no exclusivity. This move will make MS face a shit-storm. The gamers are not taking well to tomb raider (timed) exclusivity. It is accepted that you can have exclusive gaming franchise. But you should not most certainly take a multi-platform and try to make it exclusive.
Also since Mojang is a European company MS can no doubt use some of it's $90+ billion stockpile of money outside America without getting hit for taxes.