I don't get it either...anyone wanna buy these 15lb wheels I've got so I can go and get some "bling" on the car...I'm just kidding of course.
People go with lightweight wheels because dropping rotational mass is a huge acceleration improver. Supposedly for every pound of rotational mass lost equals approximately 8lbs. of dead weight.
My wheel/tire combo weighed 6 lbs less per corner. So for each corner I dropped 48lbs "theoretically".
There is a tradeoff when going with a larger lighter wheel as the mass is spread further from center.
Optimal package would be the same diameter wheel/tire but less weight.