CBSE has provided some excellent problems called as Exemplar problems on the NCERT website.
Because these problems are made by the authors of NCERT textbooks, you must do them!
Reason - the authors of NCERT textbooks are none other than the professors at the renowned research institutions including IITs and so, solving them becomes must!
Note that it is also advisable to keep correct solutions to these problems (especially for Physics and Chemistry).
The problems of chemistry Exemplar problems (especially inorganic chemistry) are really important and a serious aspirant should not ignore them!
The exemplar problems are so good (and relevant for competitive examinations like JEE Main and JEE Advanced) that Arihant has created all new series of Exemplar problems solutions.