Hi @GianLuke , I have never done the dumbbell TJ so I can’t really give much insights. But I would say usually, changing up the reps and sets enables you to lift heavier for shorter period of time thus emphasising on developing strength. And lower reps for longer sets would work on muscle endurance and hypertrophy.
For example, there is a very popular hypertrophy training called German Volume Training where you essentially do 10 sets of 10 reps at 60% of your 1 RM.
I think the Coach here is trying to mix up your training in order to condition your muscles in different way and enabling you to go up in numbers for certain movements. Also using endurance training for complementary lifts.
This way you’re exposing your body to the 3 mains style of training: strength, hypertrophy and endurance.