A nice bonus for those that like clients.
Personally, I hate them. I hardly buy anything on Steam now: instead I buy my games on GOG and Itch, because I can download and keep install files.
I did try the GOG launcher once and hated it so uninstalled it pretty soon after. I couldn't see that it did anything I wanted, but it did things I didn't want (e.g. "achievements", which I have never understood - they're the reason I had to disable the Steam overlay, so I wouldn't ever see them).
One big issue with launchers for me, apart from the clutter of unnecessary software, is that they are only made for certain operating systems. Support for older ones disappears. Why does that matter?
I have one computer that uses Linux. I have another laptop that has WinXP. I use the WinXP one for writing first drafts of my novels, stood up (it's on top of a unit). The WinXP laptop isn't connected to the Internet in any way - partly for security, partly so I have no distractions. There's nothing installed on it apart from a distraction-free plain text editor for writing. Suppose I wanted to install an old game on it though, such as HoMM2, for when I go on a writing retreat and take it with me and want something to play in the evenings? Ditto the Linux laptop.
Steam client on the WinXP laptop: I can't even find the specs for installing Steam, but I bet it would refuse to install on XP now. Even so, that laptop has no Internet connection. I would have no way of adding games that I own on Steam, that were written for XP (and may not even run on later operating systems) onto my XP laptop. Whereas an offline installer would have let me transfer the game on a USB stick and install it on my laptop.
Steam client on the Linux laptop: I could install the Steam client, see my Linux Steam games, install the game, then make Steam go into offline mode (after remembering to run the game first). It's more of a faff than just using an offline installer.
GOG client on the WinXP laptop: again, the client probably wouldn't install on XP (and if it does now, I bet it won't in the future). Even so, that laptop has no Internet connection. I would have no way of adding games that I own on GOG via the client, even though those old games were written for XP. [Luckily the offline installers I use let me download the game on my main desktop and transfer and install it on the WinXP laptop via a USB stick and install it on my laptop.]
GOG client on the Linux laptop: I don't think there even is one? Offline installers are the only option (though luckily my choice anyway.]
In each case, offline installers work; client installers either don't work, or cause problems, and even if they worked, they don't do anything I want that I couldn't do better with offline installers, though they do introduce features that would irritate me.
