Do spark plugs really causes a car to burn less gas?

Ok first off, most cars today use gasoline engines and not diesel engines so that has to be addressed as a factor (you cant run a diesel engine on spark plugs they use GLOW plugs) so assuming you are using a gasoline engine and not diesel then we can continue

SO assuming you are using a gasoline engine that uses SPARK plugs in the first place I do not understand the question.

BUT, Im assuming once again that you are wondering whether the different brands and or changing them out will yield better mpg

The answer to that is yes on all counts.

New spark plugs provide a better spark than older ones and as such increase the ratio of fuel actually burned inside the engine and not wasted. So it increases the mpg there

The difference between brands are many and the materials each brand uses also varies. But your car comes with normally piss poor plugs to start with so picking a relatively cheap and well made product can give you the same results as your typical run of the mill OEM product. I would take the extra money and buy a more higher end model but not the most expensive ones. The reason is that the higher end spark plugs last longer in between changes, increase horsepower, increase mpg, and are made from better quality. They are better all the way around. I hope that answered your question