Yeah, houses are mostly for show. No extra storage. You can potentially do some gardening or chocobo training, but those could also be done with an FC house if you have the right permissions(and even an apartment lets you use the stables).

As for the buying, open plots are rare on most servers. If you do see one, you still have to have the cash on hand to make the purchase(with even a small one being 6ish million gil). And you would likely be racing the clock, as there are others who also want a house but havent been able to get one. You also need to meet some arbitrary level and GC rank requirements.

But yeah, mostly just for show. Some people like to have a private place all their own, and some people really like to decorate(and im sure there are a few who bought a house just for the "status" of having one).