Blueprint mode is a building mode which offers additional graphical support like weapon's arc, directional cues, etc. Blueprint mode also reduces lag while building because fewer checks and simulations are run.
Blueprint mode will show additional graphical cues to support building. Parts if they are coded with graphical assistance can display weapon's arc, directional cues, red flag signs related to connectivity between parts, buff values and signs, prohibited areas, etc. Modded assistance can have warning signs, part distance/connectivity/prohibition. Further assistance is provided for warp efficiency, crew pathing and supply pathing. Further blueprint mode offers means to import silhouette images to auto generate ship plane, and also, disjointed ships can be saved in blueprint mode. Importing ships also allows to troubleshoot or overcome broken save ship files due to incompatibility between game versions or other broken save mod reasons.
Blueprint mode is the default mode when making a new ship in creative mode. Blueprint mode also allows the builder to make changes that will be applied later.