Not sure if you're actually asking for a simplified explanation, but here's one anyway:

Spider webs are made up of complicated parts, and different spiders use different parts. When a spider eats another spider's web, it breaks the complicated parts (big molecules) into simple ones (small molecules) that it can use to rebuild its own web parts. This is exactly how our own bodies process the food we eat.

It's like how you couldn't build one model of car using only fully assembled parts from a different model of car, but if you disassembled all the parts and melted down and recast the metal, you could make virtually any car.