The Tool Cupboard is essential for any base because it prevents people who are not authorized from upgrading building blocks and placing and picking up deployables within a 25-meter radius (around 9 foundation blocks) from the cupboard. If you press 'E' on the cupboard you can authorize yourself so you are able to build in this area. If you hold 'E' on the cupboard you can clear the list of players authorized including yourself. Any player authorized from the cupboard will not be targeted by any flame turrets or shotgun traps within the cupboard's radius.
Tool Cupboards can be locked with Key and Code Locks in order to disallow players from authorizing themselves without a passcode.