Your performance in 12th board exams is extremely deterministic in its own right. It is something ou carry on your professional profile for a long time. From getting admission to a college to applying for a job, everyone will ask you your board exam score in class 12. Even the IIMs give significant weightage to board exam score of a candidate in class 10 and 12.
This is not to say that those who score poorly fail in life altogether. If you have the determination to succeed, a number on a piece of paper won't stop you. However, a good score eases things which can otherwise become tedious.