Sometimes because you want users to be able to change workstations, sometimes because you want a highly specific environment outside of the user's control (it can reset on each connection), sometimes because you want nothing to be kept locally. Eg, the country somebody works in is untrustworthy, so they access everything somewhere remote and safe.
virtual desktops on demand tends to be run on servers with GPUs and in general prefers server side GPU rendering because it's meant to work with any client which can access it even if it's has an extremely weak GPU
and if you have no complex rendering requirements then often it's a much better choice to place the network gap in the GUI toolkit instead of the DM as this tends to work way better, in this case you do need a thin client on the other side, but so do you need for X11 remote (the client needs to run X11) so it's kinda not that difference. And today the easiest way to ship thin clients happens to be JS/WebGPU, which is how we have stuff like GTKs webrender backend.
