Rep ranges are more about practicality than anything else. Consider a really hard set in each of these rep ranges:
- 1-6: Weights are very heavy, warm-ups can take a while, can be hard on recovery and the joints over time
- 15-30: Warm-ups are quick, but each set is nauseating and likely to be limited by your lungs more than your muscles
For most exercises, sets of 8-12 present a nice medium, which is why they’re the go-to choice for hypertrophy programs everywhere.
That said, for some exercises it can be more or less practical to use different ranges. For example, deadlifts are quite technically challenging, and I’ve never coached an athlete who can maintain good deadlift form after 6-7 reps. As a result, I almost never program deadlifts above 6 reps. In contrast, some of my athletes complain about some joint soreness if pushing exercises like leg extensions and lateral raises very heavy. Thus, I program those exercises in the 10-15 range mostly.
Lastly, learning to program for yourself is absolutely a worthwhile endeavour, but one needs some rules/constraints to not totally fly off the wall when getting started. This article @TrainForPain shared is really good
I hope that helps!