The main buffer for immediate sub-second changes in load is the rotating inertia of electricity generators (e.g. spinning turbines).
When the total power load of the network exceeds the power generation, the immediate load is taken from generators and they slow down. Alternatively if the total load drops, the turbines speed up as they have more mechanical power pushed into them than is being absorbed by the network.
You might think it would be easier to droop the Voltage (lower the load), but for a variety of reasons it works out much better to let the frequency drop (e.g. system frequency can be reliably measured over sub-second periods, frequency is the easily measured everywhere on the network independent of phase, simple control systems before there were electronics).
The whole network relies on this for control purposes. If a network goes out of synchronisation, very very bad things happen - designing for big electricity is true engineering :) e.g. hundreds of millions of people go without power for many hours e.g. design failures in substations and commercial loads can explode due to ginormous power changes!!!
