I have been involved in cases where employees were let go due to the fact that they downloaded pron, and redistributed it.
Reasons for filters..
Bandwidth costs money.
Company is liable for what happens on it's computers
Employees WILL waste time on facebook, chat, or beyond when they SHOULD be working
Protect Computers from dangerous websites to cut IT costs
Why not to filter..
Whiney employees think that they have the right to surf anywhere anytime and come up with dozens of rationalizations to support it.
As to that lame excuse of, "I got all my work done"... what are you, in high school? If you said to me, I got all my work done, I'd send you home and not pay you for that time. As long as I'm paying you, you're working, finding work, or coming up with new projects to create work, otherwise, you're simply not needed, and you're gone.