With the traditional monitors it's just a way of a providing a convenient USB hub close to your keyboard/mouse. So you can plug your keyboard, mouse, usb keys in there without having go to below the table to fiddle with your PC.
Usually those ports are USB2 so I wouldn't use them for anything that requires speed, like external HDDs or similar.
As mentioned earlier in this thread, some newer monitors has USB-C, but I haven't had any experience with those. The only USB-C I have used is at work with a USB-C hub and it is not really anything I would recommend at all, it's quite unstable has hiccup's at the worst possible times.