In proportion spiders web strands are said to be stronger than steel. That is said to be true.
It was also once calculated that spiders get rid of so much bacteria, that if all the spiders in the world were dead, then the human race would very soon die out because of disease. Whether that is true is a question, perhaps it was a rumour started by someone who likes spiders. : -)
I think it's a build up of dust on spiders webs. When the webs are new and in use (by the spider) they are very difficult to spot. It's when the spider has moved on ( to another corner of the room?!!) and dust builds up on it that you spot it.