This’ll of course depend on the species, but usually the super long ones that people walk in to are the strands they use to get to a spot of their own after hatching. Many spiders just sort of yeet themselves to the wind after they hatch, attached with just that one single strand – that way all the hatchlings don’t just build their webs in the same spot. A huge percentage of them don’t make it, but that’s the r reproductive strategy for you