There's a lot to consider like the cost of the enclosure, and whether you need two computers. A lot of households might need two computers, so this wouldn't make any sense. You might be better off buying a full gaming desktop and just have your family share the laptop/desktop. For single people, like students, or younger people, this could be a laptop you use for school/work, and then you have the gpu enclosure at home.
It's pretty niche for me. I think the best case is if you have a laptop with a decent CPU and a good deal of RAM, but not a decent GPU. You can use it for work, productivity wherever, and then take it home to your desk where you have the enclosure setup with a monitor, keyboard and mouse to sit down and do gaming, or whatever requires a more serious GPU. But if you're buying a new laptop anyway, and you know you want to game, maybe a gaming laptop is the best solution. It's far less upgradeable, but maybe the idea of upgrading a GPU in an enclosure is pointless as well. Really depends on how hampered the GPUs are by the thunderbolt 3 connection. Even if thunderbolt 3 is sufficient, you may have to be the type of person that would want to upgrade gpus frequently to make this worthwhile over just buying a real gaming laptop.
There could still be business on the Mac-user side, because they just don't offer gaming macbooks, so having a macbook + external gpu would be an option.