During a torrential rain, even in tents with a waterproof foil the water can get inside. The water can leak along stitches. To prevent this, the stitches must be bonded or vulcanized. Vulcanized stitches resist more.
After the pieces of fabric for the outer foil were cut and sewn one to another , under the stitch is applied an isolating layer to prevent water getting in.
Cheap and low quality tents don`t have such a layer. It’s good, that before purchase, to check if the inside of the outer foil and along stitches there is a waterproof layer. In this way, you can be sure that water will not pass the outer foil even during a thunderstorm. If you are putting your clothes outside to dry a hole day from the few days spent in the camping, your vacation is not so fun anymore.