Borderlands 3 I bought when it came out on Steam, I believe in start of Steam release it was on sale. Else I wouldn't have picked it up, simple as that. They release DLCs for the game, so that's nice, and keeps it a little bit alive I suppose.
Guild Wars 2 recently launched on Steam (like 8 years after release). I cannot merge my Guild Wars 2 account to Steam though, so its useless to me.
Humblebundle, in contrast to platforms like EGS, yields you serials you can apply to a launcher, usually Steam. That's convenient, but I suppose it isn't for people who don't want to use Steam.
Though I'm a heavy Steam user, so Steam being the defacto standard is fine with me.
I don't want another launcher, therefore I either use Steam or use Lutris to abstract all other launchers (and emulators and such).
Compare to Iron Harvest, for example.
You could invest in the development via Kickstarter. Which boils down to making the project happen (due to lack of otherwise investors aka due to crowdfunding). Now, basically right after release, you can buy the game from Steam, too, but you pay full price for it (in contrast to the crowdfunders).
Contrast this to EGS. Stuff gets released on Steam many months later, say 6 months or a year, due to EGS getting the sack of money for being the sole distributor. Then, they release for full price on Steam. Why would I pay full price for a 6-12 month old game? I won't.