I don't really care about Gears of War, but Microsoft putting a new-ass game in one of their evergreen franchises on Steam is kind of a bombshell.
Basically, yeah. They want the PC crowd to look on them positively for probably the first time in a decade because they have a bunch of stuff they're bringing in the coming years, but this is still good news.MS games are on Steam because all of this is to mend all the bridges they have broken for more than a decade now.
They want to show the PC gamers that MS also cares about them and that they should subscribe to their gaming services. They wouldnt recover their damaged brand value with partnering with EGS but they would by partnering with Steam.
It is that simple.