I still use LXDE, the predecessor to LXQT, on my daily driver: a Thinkpad T420 (~12 years old) with 8GB RAM.

Low resource usage is the main reason I use LXDE. I often need to run several (4+) VMs, each of which can eat up to 2GB RAM. (These are headless VMs running specialist networking software). When running all of them I need to be really, really careful not to use up all my RAM because if I do, my system just freezes. Everything is stuck - cursor, clock, applications, etc. At this point my only choice is to reboot.

I don't know if this is due to a poorly-configured system (isn't swap supposed to prevent this?), but it certainly makes me much, more aware of memory usage.

It's also nice to be able to use the same base system (Debian + LXDE) on my modern T420 as well as a Pentium 3 laptop with 128MB RAM. I don't think I'd be able to say the same with KDE or Gnome!