Beavers(of which we hae a surplus here in the north western U.S.) are simply building homes when the build dams. The actual "lodges) they live in require an underater approcah ind entrance, hense the building of the dams first to back up the water deep enough to provide the entrances. The actual lodge can be quite large (pardon the near homophone) on the inside, accomodating 3 or 4 adults and, depending on the season numerous kits (young ones).
Our western beavers can reach 70 pounds in size and can be very destructive... they are considered pests and can be legally hunted. Though I've never tried it, some say the beaver's tail is good eating when skinned out.
Many animal's behaviours are "incomprehensible" to humans... whales beaching themselves stands out to the least understandable... as well as Sheep, goats and other such "herd" animals following one leader, which humans can redirect at will. Additionally, among other traits, why do certain dog breeds (Australian Shepherd for one) innately herd other creatures... no training needed, while other dog breeds simply ignore attempts at such training... hmmmm?