Every time you walk into the gym you know exactly which exercises you’re going to crush. You’ve certainly got your sets and rep scheme figured out. Maybe you are looking more big picture and have your entire training for the week organized. Or even a detailed spreadsheet that lays out your next three months.
Regardless of how detailed your training may be, if you are spending more time staring at the hot girl in the yoga pants doing her mobility work than you are watching the clock between sets, you are doing yourself a huge disservice. Monitoring and adhering to rest periods is as critical to developing specific strength qualities and, therefore, to achieving your goals as is exercise selection and long term periodization.
Here is a quick example: Load a bar with 185 pounds and perform 5 sets of 10 reps of the back squat with 3 minutes rest between sets. For a fairly strong guy, this isn’t going to even feel like much of a workout. Now take the same guy and have him do the same workout except allow only 30 seconds between sets and he’ll probably fail by set 3.