Well,for flavor,balance seems to be the key.
Think about a steak,chop,chicken piece you might just steam until done.
Some folks would cook with some salt to drive the flavor,might add some pepper for some back heat,might add a little sweet to balance the heat.
Mix these in a small bowl and taste them.
Is it something you would sprinkle on a piece of meat?
If something is out of balance,add what it needs.
Seems like lots of folks like a little garlic for the next taste,so add a little granulated garlic.
Some folks like a little onion,so they add a little onion powder as their next spice.
If you want more sweet,add some white /brown sugar to the rub -for short cooks.
Long cooks,add some Turbinado.
If you wish to stimulate front,mid,and back of mouth heat receptors-add red,black,and white pepper.
If you have one spice you want to be your signature,add a little of it.
More folks end up adding something objectional to their flavor profile,rather than keeping it simple.
Just my $0.02