Secondary launchers for a game can be rather irritating. Clicking play via Steam or Epic and then having to click play again is a champagne problem, if we're being honest, but it's one that 2K has gone ahead and removed for us, making our PC-playing lives that much easier.
In a blog post on 2K's site, you'll see that some games are affected differently. Games like Bioshock Infinite and Marvel's Midnight Suns will just launch, whereas others like XCOM 2 require a bit of extra work, but the heavy lifting has been done by 2K.
If you're not seeing the changes, it's recommended you restart Steam or your PC if you're playing on Epic. Also, if you want to remove the 2K Launcher files as well, you can go ahead and do that by following the guide here.