Why does spider man have a web?

Answer 1

Because he was bitten by a spider and Spiders spin webs hence the name Spider-Man.

Another answer:

In the spiderman movies, the webbing was a part of his initial powers. Meaning the webbing comes from his body and not a device.

In the spiderman comics, he gained a natural understanding of the makeup of spider webs after being bit by the spider. He then created his own formula to help him fight crime. Which can be used to swing from, tie up crooks, or even as a blunt projectile. Comment from another user regarding the above answer: When I first saw the movie, I was not happy with the change they made as to the origin of his webbing. I always enjoyed knowing that although his body was made stronger by the spider bite, he also had a brilliant scientific mind to go with it. Thus inventing the unique spiderman web formula and shooting mechanisms. Since that time, I have grown to accept the movie version as well. If you are going to gain powers of a spider to the point of wall-crawling, it would make sense that he would also gain web-shooting.