AskZilla: Without knowing the business or type of business you're talking about, it's difficult to be precise. Some possibilities include:
1. To protect them from liability. They may prevent users from accessing content which is of questionable legality.
2. To protect them from customer complaints. They may prevent users from accessing content that some of their customers might find objectionable.
3. To preserve network performance. They may have noticed that users who access certain sites use most of the bandwidth, so they block those sites to preserve network performance for other users.
4. They may have a commercial deal with a competing site. For example, they block access to site X because site Y, that contains similar content, pays the business to do so.
5. To give workers fewer distractions from sites that are not work-related.
Mainly because it is their network and they can do with it as they wish. If you are working for the company, then they want to make sure they are getting their money's worth of work, for what they are paying you.
How about because they are paying you to work while you use their computer resources, and not just sit there watching porn or youtube videos! It's really that simple.
Why not? It is THEIR wifi. None of your business.