Secondary cost element create for internal purpose of 
controlling area.  This is for service provide to one cost 
center to others cost centers.

Expamle:-
In your office you make the CANTEEN is cost center. Canteen 
is provides the service to other cost centers like TEA, 
Coffee, etc. You allocated the TEA, Coffee Amount to others 
cost center that thru secondary cost element.

If I am wrong so please correct me.

Thanking

It is used for co. cost allocation purpose. I just wanna 
add one thing in the answer above, that is cost center 
provides internal activity to other cost center, and that 
internal activity is distributed by a secondary cost 
element.

Secondary cost element capture operation costs, overheads