Hey Nikhil,
Doing 12th can be both beneficial and wastage of time it depends on what you want from your life .
The thing you want to pursue if not meeting the requirements and eligiblity criteria and you have the strong desire to pursue same then doing 12th might be a second chance to pursue your dreams and if it is meeting the requirements then I think doing 12th again would be a wastage of time
Doing 12th again is a option for those who haven't cleared their 12th at one time and the ones who have cleared it but still want to improve the score then they go for improvement .
So it's totally depend on you what you are expecting from you and from your life
Many good luck to you!