This is incredibly niche but say you're building a plane and you have the fuel tank set on jet fuel with fuel lines on, you finish building you're plane but OH NO you only have 1 fuel tank set to those settings and the rest are empty with no fuel lines, now u have to go back and change all of them.
My idea is that you can have a setting thing unique to each craft so that when you go and grab a new part from the part menu it already has the settings you need. Is this unnecessary? Probably. Is this going to save time? Probably not. Is this because I'm really forgetful? Yes.