Standard Service Catalog

Dear Experts,

Through AC03,we prepare Service master Records and through ML01 we prepare Standard Service catalog.

Now whats the use of Standard Service catalog?

and where do we use it?Do we use it for Service Master Record creation?

I heard it reduces data duplication.But did not understand where does data duplication happen in which tcode?

I saw that we have create a hierarchical structure in Standard Service catalog with descriptions.But whats the use of it.

I am Confused...Can can Expert please clear my Confusion.

Kind Regards

Sajid