Knowing PH & EC allows you to dial in and take the guess work out of feeding.
Think about baking a cake, you can certainly eyeball your ingredients and get close to your desired measurements. More often then not you'll bake a pretty good cake. But using exact measurements allows you to make specific changes and "dial in".
Eyeballing will only get you so far. IMO a solid PH pen & TDS meter are worth the ~ $50 - $100 investment.