Cartridge cases [and spouts] may not provide quite as much of an accurate and versatile option for making calibrated adjustments to the powder charge or filler if you ever decide to try another load.
The same applies if you ever try to work up a load for a conical bullet or using different powders
And if you ever buy another gun that has much more or less room in the chambers, then you won't need to hunt around for or to construct the right size of a fixed volume powder measure from a cartridge case.
A graduated powder measure may not be absolutely necessary to shoot black powder guns, but it doesn't hurt to use one or to at least have one on hand for experimentation or a contingency.
I use a plastic funnel when filling the chambers, not because I need to but because it helps me to not spill as much powder as I would otherwise.
So sometimes there's an actual benefit for using some luxury item.