Solving model papers, sample papers, giving mock tests is really important as far as your academic performance is concerned since it helps you to judge yourself well as where do you stand, what portion you've covered or mastered, what else you need to work on, etc factors.

You cab refer to this site to get the model papers of BSEB..

https://school.careers360.com/articles/bihar-board-12th-model-paper

Best wishes. Thank you.