A ship can only realize a fit it can support. Reactors provide electrical output, active systems draw power when their role is in use, and cooling absorbs the heat burden created by the operating ship. Power or thermal shortfalls reduce operational efficiency.
Some systems draw continuously; profession hardware, drives, weapons, and other role-specific equipment draw when the relevant operation needs them. Available reactor output is compared against that current load.
Cooling capacity is compared against the ship's current thermal burden. Falling short reduces thermal efficiency and can compound the cost of an aggressive fit.
The Weapons / Shields / Engines control changes combat emphasis inside the engagement: weapon pressure, shield recovery, and maneuver/range control. It does not replace the underlying reactor and cooling constraints of the physical ship.