What is the best reason why the spider lines its burrow with silk?

Answer 1

Lining a burrow with silk has several advantages. Which one is the most important may depend on other factors, such as the weather.

In soil that is very stable and unlikely to cave in, burrow linings are probably most important for helping the spider get a good grip as it moves in its burrow, and for giving the spider a good place to hang its egg sac after it has laid a clutch of eggs.

In soil that is sandy or in soil that gets shifty when wet, the biggest advantage to the spider is that the webbing helps keep the tunnel from caving in.

Trapdoor Spiders sometimes get attacked by wasps or other animals that try to pull their door open. The spider grabs her side of the silk-covered door with her fangs and hangs on tightly to the burrow wall with her eight feet. Without a burrow lining to give the spider's claws a good grip, it would be very much easier to pull her out.

Answer 2

Spiders use their silk for making webs of course! They also use silk to wrap around their prey.