Yep, got Sky UI... Zero, why would I want a basement in all houses?
More for convenience. If you haven't noticed by now, not all homes are equipped similarly. This mod has gives you all the tools you need that you can access from any house once you've unlocked it. So you have a lots of storage, an alchemy lab, smithing facilities ans so on.
Without the mod, if I want to make potions I have to go to Breezehome as that's where I've stored all my alchemy ingredients. But it doesn't have an enchanting table - for that I have to go to another home. But the alchemy table there doesn't have a convenient satchel at the table there, unlike Breezehome. I also collect the weapons given me when I become Thane, or unusual armour that I don't want to sell. The basement gives me lots of display racks & mannequins to display them.
And so on.