why we want to use index in sql

Let us imagine the index of a book or a dictionary , if we don't have index, we need take a lot of time to find the specific information. In sql server, we call this problem as scan, SQL SERVER engine need go through from the first page to the last page of an offset by offset or row by row. However, if we have index, even clustered index, we can go to the specific node and fetching the information needed