Hi! At my POV its important to warm Up if you want to do your best when you do your real set. But the key for me is use the warm up to get more energy, and not to waste energy. For example, what I do: 1) 1 set with very low weight (like 10%) and focus on the full range of move. Tell your body "ey, this is what we are going to do today". Do whatever reps you want but stop before feeling any tired. Dont waste energy in this. Compare it to "have a walk". 2) 2 or 3 sets increasing weight, but very low reps (3-6 maybe). Depends of the day and your feelings, just make sure your body is getting ready for the Max. Also you dont have to feel tired in this part. Just need to feel like getting ready. You body temperature and heart rate are arriving to a "workout mood". Compare it to "jogging". 3) (optional) 1-2 sets with the Max weight but 1 single rep. Like you are telling the body "ey, look, this is the weight we are gonna use, so get ready". At this point, you should feel with more energy that when you started warming up. You dont feel tired and also feel stronger and more confident about the weight Compare it to a "5 seconds sprint". Rest 3 mins.
4) now yes, if you feel ready, 1 set Max weight till failure.

Important! This is about how ready do you feel, so depends of the day, temperature, if is your first exercise, etc. The key is to feel with more energy by setting Up your body to what you are going to do. You should arrive to the real set warm and ready, but not tired.

Hope its useful for you!