UK Electricity Generation Map

Dispatchable power is probably a more neutral term than generation. The reason battery is included in tools like this is that there is a rapidly growing, non-trivial amount of dispatchable battery power on grids. Anything that excludes that is kind of not giving you the full picture. And of course this is widely expected to vastly increase in amount of dispatchable GW over the next years. It's early days.

Dispatchable power is what matters if you are managing a grid. If you have lots of dispatchable power, you can deal with peaks and dips (both are valuable) in demand very rapidly and relatively cheaply. Long term, daily, and seasonal trends are fairly predictable. Even the weather is short term fairly predictable.

Battery adds a lot of predictability to grids. If you have hours to plan for it, you can bring online gas if needed (coal and nuclear are not typically kept around in a mothballed state for this). If it's seconds, you are screwed because that is non dispatchable power (you need to plan to have it and spend money to get it online). That's the gap batteries fill.

Anything with flywheels (gas, coal plants, etc.) needs time to (literally) spin up. And before that happens you first need to generate steam by boiling a large amount of cold water. Some gas plants can start relatively quickly but then run less efficiently. You can trade off dispatchability for efficiency. It means expending a lot of fuel just to get the thing started that otherwise delivers no power. They provide a lot of power once they are up and running but they are kind of useless when you need that power right away. And the more often you shut them down, the more you lose on spinning them up again.