A tarp can serve as a tent footprint. It will still act as a waterproof and durable barrier to protect your tent floor. Just be sure it is the right size to avoid funneling more water towards your tent in the event of a storm. Cutting a tarp to size results in a fraying mess, but you can buy one appropriately sized for your structure or fold it to the correct size. I wouldn’t recommend a traditional tarp for backpacking as they’re relatively heavy.