This is something I've wondered about for years so finally I'm asking. Why do people use purchased footprints for their tents? Or make them from Tyvek or other special fabrics?

My understanding of the footprint is it helps prevent tent wear by preventing the tent floor rubbing in the dirt when one is in the tent, preventing water from soaking through, and preventing minor twigs or pebbles from poking through.

I've always used a Hefty Lawn bag cut down the sides and folded open lengthwise. It's big enough to cover the area of the tent that I will be putting weight on, is waterproof, and being a lawn bag is puncture resistant. Plus they're cheap. If it sticks out from the tent, I shove it under the tent. I see no particular advantage for it to match the tent perfectly; it just needs to protect the area of wear.

I have no idea what it weighs but it isn't much. One bag can be re-used many times, probably as many as most fabrics.

Am I missing something? I know, do whatever you want, HYOH, etc. But is there an advantage to Tyvek or custom footprints?