Why do we need dp charges?

Why do we need dp charges? Why are they separate & not part of brokerage charges? Why they need to be per script per day?

Quoting from here.
DP charges include Depository charges and Depository Participant (DP) charges. The Depositories are NSDL & CDSL and Depository participants are members of Depositories with whom you open your demat account (Eg: IL&FS). DP charges are applicable whenever you sell any shares from your demat account. This charge is a source of revenue to the Depositories & its participants. Similar to how Exchanges charge Transaction fee, Brokers charge brokerage, Depositories & DP’s charge DP charge.

Brokerage and depositories are two separate entities.

Each scrip you sell counts as a transaction. Even if you sell the same scrip multiple times in a day you will just be charged once.