They had to do this because instead of regulatory pressure, they were feeling the opposite: Competitive options in the PC market. With Steam's high cut, any publisher large enough to make their own launcher should would be incredibly stupid not to do so, which is why Activision, EA, Ubisoft, Bethesda, Epic, etc. all have their own launchers now. And some of them have, or could, start selling third party titles as well.

Valve is presumably hoping giving these large publishers a deal will cause them to reevaluate the effort in developing and maintaining their own storefronts.

...In otherwords, it's also an anticompetitive play, it's just the one that makes sense in Valve's particular monopoly space.