Like other forms of silk it is very strong for its mass and, in many cases, has good elastic properties - however it is not a particularly efficient industrial resource given that most spiders coat their silk with adhesive and spin it into webs (although there are spiders who use it for tripwires or other forms of engineering), also spiders tend to be solitary carnivores and therefore hard to farm.
These webs are generally designed to catch creatures the spider would consider prey - generally other small arthropods - and are rarely strong enough to act as anything other than a distraction to humans: even those species that aim to trap small birds and mammals don't spin a significantly impeding web. Of course in settings with bigger spiders and/or smaller PCs this may not always apply. In some cases, groups of spiders may spin communal webs, but whilst visually impressive these are no more troublesome than regular webs except as regards their ability to block line of sight. Abandoned webs eventually become fouled with dust and turn into "cobwebs" - those greyish strands all too familar to people who don't undertake housework.
For reference, spiders can built webs in microgravity - after a period of adaption - although their quality is somewhat patchy. The adminsitration of various drugs has also been observed to have effects on the design and integrity of spider's webs.
Historically balls of spider web were used to dress wounds - this turns out to have some scientific basis given that residual glue on the web is rich in vitamin K which is important in blood clotting. Synthetic spider silk remains an objective for materials science and there is also work in progress to produce it from other species by genetic engineering.
Game and Story Use
- Besides the synthetic silk, biothaumaturgy or some form of biotechnology might create a modified spider that could spin web on an industrial process - probably looking more like a tick with vestigial limbs and probably fed directly from a large animal like a cow or a bag of pureed meat.
- Fantasy giant spider silk anyone? Probably good for catching human sized prey, but possibly also used for industrial applications.