A car employs an engine and transmission system to power it, while it rolls on wheels supported by a suspension system. For the sake of supporting the engine and other systems, a support structure is used. One of these categories of support structure thats very widely used is called monocoque construction. In case of the monocoque construction, the skin formed out of metal is employed to bear stresses. In principle, its much like an eggs shell. The formed sheets are ultimately welded to strength and bear tensile, bending and torsional forces that act on it.
The sheet thickness thats employed to form various sections and welding technology thats used etc. are important decisions as it can affect the cars handling capability and safety that it provides to the passengers.
A monocoque construction is much lighter in comparison and the amount of material that it requires to provide significant strength to the car is much less, therefore from manufacturability point of view, its economical to produce.
All monocoque cars can be made lighter with the use of better materials and iterative computer analysis, so, another great benefit that it offers is the high fuel efficiency due to the low weight that it can achieve through inherent design that it has.
However, monocoque constructions aren't without its demerits. Development time that it requires is comparatively longer due to the complexity it involves and requires more rigorous testing and a more rigorous iterative design analysis. This form is not very well-suited for off-road vehicles as rigidity and torsional stiffness is not very easy to achieve without making the car top-heavy.