Sporting my new rear pannier baskets, (my kids call them granny baskets).
Most days it's nothing more than my change of clothes, tool kit, extra batteries, extra water bottle, travel mug of coffee and my man-purse.
On the days my wife calls and says, "Can you pick up such and such from the store?" I've still got room for milk and eggs in there.
I play soccer in a Friday night, adult league. I head to my games straight from work, so I have to bring my goalie kit and shoes with me.
Some nights I go
geocaching which can involve a lot of bushwhacking through greenbriar and poison ivy. Those nights I've got my hiking boots, trail gaiters, headlamp, snake hook and sometimes an ammo can.
So there are lots of uses for most any bike mounted container.