Two main reasons come up for splitting. First, if sides hit a day or two after the shot and then fade, they're probably from the peak, so smaller more frequent doses can avoid that. This helps mainly if you're still trying to lose weight or control hunger and want to go higher but sides are blocking it. Second, if hunger only returns around day five or six because the low point isn't enough to keep it down, splitting raises the minimum level twice a week. You could also just raise the total dose instead. For most the six-day half-life isn't very noticeable anyway. The big plus of once a week is how simple it is and how hard it is to mess up.
I've picked up bits of both arguments. The studies showed strong results from one shot a week, yet the roughly six-day half-life might mean weaker effects toward the end of the week, and splitting could keep peak blood levels lower and therefore cut down on sides. What I'd like is someone pulling those points together, maybe saying one weekly shot suits certain people or aims while splitting works better for others or different aims. Or maybe I'm missing something obvious and someone can set me straight.