I think that the heads on watering cans can be considered a type of flow restrictor. I use a one gallon watering can in my garden, and the cap at the end of the spout has several holes that allow just a small amount of water through at a time.
Using a garden hose can be hard on plants if you let the water come out full force. That's why I prefer to use a sprinkling can. It's very gentle on the flowers.