The two most important effects in a guitarist’s signal chain are distortion and delay. And if you derive your tone strictly from the amp—whether it’s squeaky clean or buzzsaw nasty—then the digital delay is numero uno.
Another usage of a delay that I have found is that of recording wider than usual signals (without having to record doubles of the same take). For example, some delays have 2 outputs "dry out" and "mixed out", which can be used to create 2 mono recordings/tracks of the same performance and pan them hard left and right (or whichever way that one prefers) to get a much wider sound. But for this to work, the delay time needs to be set appropriately. I usually set somewhere between 4 ms to 20 ms depending on what type of stereo width I'm going for and how it all feels!