The Sawmill is the backbone of your lumber industry. Having a good number of Sawmills is recommended in any scenario as you will need Wood for buildings and training all troop types except Ground.
As is with all Resources it is recommended to not open your Lumber boxes until you need to use it. This prevents your stockpiles from be stolen in the case that your bubble drops.
The Sawmill ends up being a building requirement for the Archer Camp.