When we are focusing on shaping and building our ideal body, every single rep counts. There is a scientific approach we can take to do that. It's not about getting in the gym and throwing weights around with no specific goal. The time you are under tension, your speed and tempo and the amount of time you rest in between your sets MATTER
IF your main goal is fat burning- then higher reps and less rest between your sets is ideal
IF your main goal is lean muscle gain- then you need lower reps, heavier weight and more rest between your sets
I personally like to do both. You can MIX these two approaches. One day focus on heavier and lower reps and the next time you hit those muscles, take the fat burning approach with them. OR, you can do both styles in the same workout. Do half your exercises with the muscle gain method and the other half with the fat burning method.