Sometimes you want to make construction plans, but not have your colonists begin work on it right away. This can be achieved by forbidding the blueprints, but becomes a hassle quickly for large projects, as the vanilla allow/forbid designators indiscriminately toggle everything including items that may be in the area.
To get around that problem, Forbid Blueprints adds two new designators that target only blueprints and frames specifically. With these, it's a breeze to toggle building for complex constructions.