Well, "adds strength" relative to what? A single-piece continuous hull is the strongest & most reliable but you can't see thruthrough it. Adding a windshield to the windshield frame of a car makes the structure much stronger than the relatively thin rectangular frame itself.
When weight is a concern, as with the beams and cross-braces on a bridge, a lot of material can be removed without significantly affecting the member's bendbending or shear limit.
