When it comes to building vast towns and cities, having to build every structure individually might get tedious. Therefore having pre-set/custom blueprints for entire structures might make it easier for players to build bigger settlements.
I’ll explain with an example; if you were building a street of two-storey houses which were similar if not identical, wouldn’t it be convenient if you could simply build the house once and copy/paste it for the entire street? The builders would still have to collect the resources for it, but the blueprint would simply be one click for the whole house rather than many clicks for the individual walls, doorways and roofs. Blueprints would be far quicker and would open the prospect of large town development to more casual players. There could perhaps even be a Steam Workshop where you could upload blueprints of your castles or other such structures for others to use, like Medieval Engineers does.
This feature is not essential nor game-changing, which is why I’d call it a Quality of Life improvement for further down the line. It would probably be more useful the further into the game you are, when you have more people to manage.
Just a suggestion. I could be convinced of its usefulness either way.