A donated dollar is one less taxable dollar, so in practice provides a means for individually diverting taxes to charities one finds agreeable.

In the US I've even heard people using this rational to avoid fuelling the military industrial complex. However a well functioning government that best served society would not have this problem. In principle it's the government's job to take care of society's needs, not the charities'.

I think the existence of Planned Parenthood is a case in point. Undoubtedly they provide a necessary service, but in what world should a pregnant teenager not have access to a doctor?

In practice charities act as a highly inefficient decentralised mechanism for providing basic needs to those that can't afford them. In some cases, the charities are stellar. In others, they come with a religious taint (e.g. access to a doctor through a church). In the worst case a charity can operate as a scam, see the untouchable tele-evangelists.