If I’m doing a new period/army or whatever it is because I want the toys for it. So I’ll plan what I need, buy it, paint it and then play with it. I will often have some idea what rules I’ll want to use, but tbh with a big of imagination you can use the same figures with any old set of rules.
I will often design scenarios specifically to use new stuff I’ve bought or built, just so they get on the table.
"Mistakes in the initial deployment cannot be rectified" - Helmuth von Moltke