A database index is a data structure that improves the speed of data retrieval operations. Indexes are necessary for high performance read operations for frequently used queries. Without indexes, database table/collection will be scanned fully to select required data that match the query statement. Full database table/collection scan is very slow operation for a huge amount of data. But indexes can increase read operation performance to point to exact records without necessity to scan all the records in storage.
Indexes are very useful and essential for database storage at the cost of additional storage space and influence on write/update/delete operations speed.
Fundamentally, indexes in MS SQL Server and MongoDB are similar. But, of course there are a couple of differences between them.