It's pretty important. The regular 10 minute warm up with your opponent is better than nothing but it's more of a formality than a real warm up. Better to hit for 20 minutes or a half hour with a friend before the match. It's not a coincidence that all pros do this. Before the US Open finals between Martin and Agassi, Martin hit for over an hour comprehensively going over all his strokes with Jose Higueras. Whereas Agassi came out groundstrokes blazing, hit 4 or 5 overheads, a few serves, and was done in about 15 or 20 minutes. Amazingly, Brad Gilbert could get back anything Agassi threw at him so long as he didn't have to move. Agassi was amazing. Never seen anybody hitting that hard. Martin was really grooving all his shots knowing all his shots would need to be working to beat Andre. Andre was just there to make sure nothing was broke that day.
Foremost, go through all your strokes, and see if any particular shot feels super off that day. If so, try to work on it. Maybe a friend can hit serves from the service line to get your returns going. Or feed you some crosscourt backhands that you can hit up the line. Or feed you short balls for you to put away. At that point, of course you're not going to develop a new stroke, it's about walking onto the court with confidence, knowing you just hit several good shots fresh on your mind. You want to go out there a sweaty and knowing that all the shots are working and win or lose, you brought everything you need with you.