I don't need it. I install games by clicking on the provided .sh installer, then click on their desktop icon to start them (or find the app using shortcuts) and all of this is seamless on Ubuntu.
This is also how we used to install and play games on Windows, before the Steam era.
If a company pretends to support Linux, they should have a Linux client, whether I need it or not. They basically offer no Linux support, and all the Linux clients that come on their store pages come from developers' willingness, not GOG's.
They sell games that work in Linux, what is there to "pretend"?
I disagree that they "should" have a Linux client, for the reasons I explained. I don't have a use for such a client, and as such, I'm happy to ignore it (and love that it isn't mandatory, unlike Steam).
I've never received support from Steam for any technical issue. I'm resourceful and if there's a way, I'll make it work; but almost every help I needed with Steam games was taken from user's forums, within and outside Steam.
Case in point: I learned browsing Steam's forums that the Linux version of The Eternal Castle is glitchy and in many cases the keyboard input doesn't work at all; one user helpfully suggested just using the Windows version using Wine or Proton; this suggestion worked.
> Itch.io has a Linux client, by the way
Good for them! I hope it's optional, since I've no use for it.
What I love about Humble Bundle and GOG is that their games (mostly) don't try to take over your games collection, unlike Steam. I do use Steam and support Valve, but I'd prefer Steam was optional.