A few benefits come to mind:
1. They will watch less TV.
2. You’ll have fewer arguments and fights with them.
3. They’ll learn to self-regulate.
Have a system – a piece of paper will do, but an app on all your phones is also a good idea – where you write down what is your point of view on what and how much weekly TV is ok. They will use this to record each and every time they watch TV how long and what did they watch. Every time they’ll see what’s your accepted “allowance” is. Each week you’ll sit together and review last week’s results.