I'm like a lot of others that probably carry way more than I need, but here's what I carry in my vest:
1-3 mouth calls, 2-4 pot calls, 3-6 strikers, hand pruners, gloves, 2 facemasks, crow call, gobbler shaker, 4-9 extra shells, scotchbrite pads, water, Thermocell (according to weather), pen, extra batteries, rangefinder, GPS, box call sometimes, Gobbler Lounge (sometimes in the afternoon), & decoys sometimes. But the #1 reason I use a vest is to have a place to put shed clothing later in the day when it warms up & #2 is a good seat cushion. A vest just makes it easier to keep up with things & less likely to forget something.