Problem 1: you have a slightly old machine (512MB or 1GB of RAM) or something like a Raspberry Pi and you want a resource-efficient but modern-looking desktop.

Problem 2: you want a VM with a DE but don't want to spend too much resources in it.

Problem 3: you have a modern computer but prefer to have a lightweight DE and let the rest of the resources be used by your applications.

Both LXDE and Razor-qt consumed about 100MB of RAM, I would expect something similar from LXQt. All the other DEs are heavier, XFCE is just a bit heavier but at least for me LXQt looks a lot nicer.

And as it was pointed out, it's a merger of LXDE and Razor-qt, so it's the complete opposite of the "many Gnomes" situation. Kudos to that.