Watt-hours are what matter. If you want to boil a kettle, you need to use a certain minimum amount of watt-hours to do it, which corresponds to, say, burning a certain amount of propane. If you care about how many times you can boil a kettle on a bottle of propane, you care about the watt-hours expended. If you had perfect insulation you could boil a kettle by applying barely any watts for a long enough time and it would take the same amount of propane.

There are situations where Watts per se matter, eg if you build a datacenters in an area that doesn't have enough spare electricity capacity to generate enough watts to cover you, and you'd end up browning out the grid. Or you have a faulty burner and can't pump enough watts into your water to boil it before the heat leaks out, no matter how long you run it.