A few reasons why
To shield givers from continuous requests by charities
(and hopefully charities redirect the resources they spend on such requests)
To enable giving small amounts in intentional ways (ex: Direct Change)
To enable giving through diverse skillsets (ex: 100% Store)
To enable a variety of charities to receive funds more efficiently (ex: )
Custom indexes may be shared - enabling promotion of productive charities and intelligent designs of giving
To streamline accounting procedures for givers which give to multiple charities
To make it easier for givers to select the most effective places to give
Considered giving seems likely to be more effective