Nothing replaces sparring, you have to go pretty hard sometimes to make the spar sessions "feel" like a fight would when a bout is coming up. And the timing, reflexes, reading, distance judging, countering can degrade when you dont keep them sharp with spar sessions.
You definitely dont wanna go into a fight and encounter anything you havent felt in training, thats why a lot of dudes go hard, especially when a fights approaching. Huge difference in fatigue and ability to relax when sparring say, 50-60% and when you go near 100%. Also gives the coach a sense of how youre gonna react and what techniques you choose when things get tough in there.