ADP Rankings are critical. Why? Because, they tell you what your opponent is going to do on average. Think of ADP Rankings like a predictor of the future, and if you were going to go buy a house right now and knew that 80% of everyone looking at said house was going to spend $350,000 even for the house. Knowing this information would give you a HUGE advantage to acquire this house, as you know what people are willing to pay and you can swoop in and: A) not overpay if you don't need to, B) Overpay just enough to land the house.
Drafting players is no different. Knowing ADP Data means knowing what your opponents are willing to spend, and it allows you to let Chris Olave fall into round 3 if your league allows it... it allows you to wait on Justin Fields in round 4 vs. go quarterback even earlier... it is the key to smashing your league mates. You should NOT draft off of ADP, but maneuver around it and get value every single time you pick.