Use of Non clustered Indexes
If a table is large and growing fast, clustered index might become too expensive to maintain since the DB server has to reshuffle all the data while rebalancing the tree, not only nodes with key values. It might significantly affect speed of data modification queries.
Clustered index stores table data right in the index (a tree node contains the whole record, not only the key). So when you rebalance the tree of this index, you also have to physically move records. In comparison, non-clustered indexes store trees separately from the main data and only use pointers to physical placement of corresponding records. So they are much cheaper to rebalance.

For most DBMS (TokuDB for MySQL/MariaDB/Percona) is an exception) there can be only one clustered index per table because it defines the physical data placement. So, if you need several indexes on a table, you have to use non-clustered ones.