The Threat, Fortitude, and Value scores for a ship, which are the sums of those values for the components in the design, are used in combination with the hull size to determine its default role.
The way it's supposed to work is if Value > Threat + Fortitude the ship gets classed as Support. Otherwise, Tiny hulls get classed as Interceptors if Threat > Fortitude or as Guardians if Fortitude > Threat, Small hulls get classed as Assault if Threat > Fortitude or Escort if Fortitude > Threat, Medium and Large get classed as Capital if Threat > Fortitude and as Escort if Fortitude > Threat, and Huge hulls get classed as Capital regardless of the relative magnitude of Threat and Fortitude.
As far as how it factors into weapon choice? I would say that the only time this factors into weapon choice is if you're trying to manipulate the computer-assigned default role into being what you want the ship's role to be.