Like a tax-break for the super rich, and with an official response that weirdly sounds like trickle-down economics - Valve is changing it's 30% profit share policy of all revenue for every game sold. Which is now 25% for games whose revenue hits $10 million USD and 20% for those that hit $50 million USD. As to why the company is making the changes now, is anyone's guess.
One could view it as a move to lure back some of the biggest publishers in the PC space, like Bethesda who eschewed Steam entirely for its own platform to release Fallout 76. Traditionally Bethesda titles like Skyrim and Fallout 4 have been some of the most popular titles on the platform.
And then there's EA with its Origin service and Ubisoft with Uplay and even Activision moving its releases to Blizzard Battle.net. As per the official announcement the change went into effect as of October 1, 2018.
The value of a large network like Steam has many benefits that are contributed to and shared by all the participants. Finding the right balance to reflect those contributions is a tricky but important factor in a well-functioning network. It’s always been apparent that successful games and their large audiences have a material impact on those network effects so making sure Steam recognizes and continues to be an attractive platform for those games is an important goal for all participants in the network.
Valve is well within it's rights to set whatever revenue share policy it feels is necessary on Steam, but it would have been nice to see smaller indie titles get some attention too.