The point of controlling what workers do is to increase the productivity, not gossip. The managers are not there to prevent you from doing something that is not related to work, so that when they catch you do something, they go ahead and gossip about it with other managers, that ain't the reason, hell they couldn't care less at all.
However, they do it so that they could increase productivity, if they are following you on CCTV and they are checking every website you are visiting, that means they are trying to just keep you working. In most cases, someone works 8 hours a day, with 1 hour break, and 2 15 minute breaks, that's how it has been everywhere I worked, that means you work 6.5 hours and during all that 6.5 hours, they want you to only work and nothing but work.
Yeah, the point is to increase the productivity of workers but to be fair, it's insane when they expect factory and warehouse workers to work the whole 6.5 hours without any 1-2 minute breaks. I have done physical labour and I know how big the pressure is to work, they expect you to carry 20kg boxes every second for 7 hours. I call it slavery. I studied UI/UX design and got a good job. Now I get a high salary and I am not watched and forced to work every single second. During the 8-hour shift, I take a 2-3 minute break any time I wish to rest my eyes and I also have 1.5 hours of paid break time.