So many of us riders enjoy a color-coordinating outfit based off our horse. Perhaps, we ascribe to color psychology and want a pair of breeches in every emotion we want to channel. Some of us encounter lifestyle changes that change our bodies' shapes and sizes often, so buying multiple sizes is a must. If you crossover disciplines, maybe you want different cuts and seat styes.
All these situations are personal and different. Thus, there is no cemented formula or answer to deduce, but I'm still willing to try.