Think of a hard plastic vessel. Maybe one liter in capacity. Inside is a super duty rubber balloon with a few pounds of pressure in it. As your pump runs, it fills the tank, compressing the air inside the balloon. If you need just a little flow, say to cut in close to the edge with a feather trigger, the balloon pushes the mix back out of the vessel so that the pump doesn't rapidly turn on and off. Rapid on and off (short cycle) is the death of these little electric pumps.
The accumulator is the life vest that we were missing for so many years.