Hey there @Ardvolf, welcome to the site!

So, you can only have one house active at a time, and there's no obvious benefit to having that much inventory space for houses. However, for players who have the inclination, that inventory space could be used to flip houses. It doesn't happen too often, but occasionally you'll see someone selling a house for significantly less than it's worth, and you could potentially profit from it by reselling it for more. It's a slight gamble though, since you have to pay a 10% fee to list a house on the market, and your listing will only last 30 days. If nobody buys the house, then you've lost some money on the fee.

Other than that I can't think of a reason to have 6 slots for houses, although sometimes endgame players like myself buy people's starter huts just for fun. :P