Most employees waste more than an hour on browsing web pages. Even worse, someone will not be able to concentrate on their work during work time.
So, to save productivity, it is necessary for organizations to block certain websites and restrict internet access.

In my opinion, things should be done from several aspects:

1. Only work-related websites are allowed during work time.
2. Destructive websites like violence, adult, shall be blocked always.
3. Downloading websites shall be blocked to save bandwidth if you are suffering from slow internet speed.

For those companies who are very strict with websites browsing, you can implement a website whitelist, by which, only websites in the whitelist can be visited.

More information, please refer to internet blocking and internet monitoring.