When you take a nice piece of waterproof material and sew through it, you punch holes through the waterproofing.
These holes need to be sealed up, and the easiest way to do that is for the manufacturer to apply a hot-melt tape over the seams of the tent. It’s rather like Sellotape which sticks when you iron it on.
2-4 season tents will feature taped seams. And many waterproof jackets and trousers will also feature taped seams. Our Wet & Dry Backpacks have taped seams.
You can’t easily apply seam tape yourself – it requires a machine set to melt the tape but not the fabric. However, you can get an aftermarket seam sealer (McNett Seam Seal) if your tent/bag develops any leaks… which it probably will eventually if you rub on the seams for years. I’ve not tried this on a garment, but the principle is the same… if you have access to the inner face of the garment itself.