The former "Extreme Makeover: Home Edition" host tells TMZ ... hardware stores are necessary if you want quarantined folks to maintain their sanity ... and he explained why home improvements help achieve that goal.

Ty says it's not just about keeping busy either. He says projects to make your residence a better space, make you happier and more productive. It's a principle he wrote about in his book, "Good Design Can Change Your Life."

Guess he was ahead of his time -- and right now most Americans have a ton of time on their hands to practice what he's preaching here.

Ty's bottom line ... a lot of good, for your future, can come out of being stuck at home now.