If a FC owns the house then yes, it lets you do airship/submarine stuff. You build airships and submarines and send them out on voyages and they come back with items. There are certain items that can only be obtained from it.

Also, outdoor furnishings. And storage for furnishings. I have an apartment, a FC house, a FC room, and a private house. Each of those has it's own storage. Someone with just an apartment can't store as many indoor furnishings or any outdoor ones. And furnishings are pretty regularly given out as limited time event rewards that later cost real money on the mogstation.

But it's primarily coveted because there are only so many houses and people like having things that not everyone has.