If we can loop through using while loop then what is the use of cursor in sql ?

Cursor is more control like pointer to traverse through the rows in table. While is precondition loop. Based onyour requirement you can opt for an option.

This article explains in details. 

 https://www.techrepublic.com/blog/the-enterprise-cloud/comparing-cursor-vs-while-loop-performance-in-sql-server-2008/