The biggest reason in my mind to use PGP is to resist the government. That sounds pretty crazy. I don't mean resist in an anarchistic or Disestablishmentarian way but in the same way that one doesn't want themselves being frisked by police daily. (Which seems to happen to me alot. Maybe its the way I look...)
The FBI, NSA, DEA and other government agencies now have the ability to wire-tap pretty much anything they want to. Most recently this has come in the form of the Omnivore and Carnivore boxes, which are installed at an ISP and filter through all the incomeing or outgoing packets to pick out those of suspected criminals. This means they read all the information passing though an ISP. That could be anything mine or your private emails, to our banking records, to this node I write, right now. Maybe thats not a terrible thing but I will feel alot safer when my web browser uses cryptographically strong encryption
The best reasons I've seen are however on the PGP home page. Here is one of many letters posted there:

Date: Mon, 18 Mar 1996 15:32:00 +0000 (GMT) >From: name and email address deleted Subject: More News from Central Europe To: Philip Zimmermann
Dear Phil,
I have been thinking of specific events that might be of use to your Congressional presentation. I am concerned that our brushes with Governments might be double-edged in that Congress might not like the idea of Human Rights groups avoiding Police investigation, even if such investigations violated Human Rights.
However we have one case where you could highlight the value of PGP to "Good" citizens, we were working with a young woman who was being pursued by Islamic extremists. She was an ethnic Muslim from Albania who had converted to Christianity and as a result had been attacked, raped and threatened persistently with further attack.
We were helping to protect her from further attack by hiding her in Hungary, and eventually we helped her travel to Holland, while in Holland she sought asylum, which was granted after the Dutch Government acknowledged that she was directly threatened with rape, harrassment and even death should her whereabouts be known to her persecutors.
Two weeks before she was granted asylum, two armed men raided our office in Hungary looking for her, they tried to bring up files on our computers but were prevented from accessing her files by PGP. They took copies of the files that they believed related to her, so any simple password or ordinary encryption would eventually have been overcome. They were prepared to take the whole computer if necessary so the only real line of defence was PGP.
Thanks to PGP her whereabouts and her life were protected. This incident and the young woman's circumstances are well documented.
We have also had other incidents where PGP protected files and so protected innocent people. If the US confirms the dubious precedent of denying privacy in a cavalier fashion by trying to deny people PGP , it will be used as a standard by which others will then engineer the outlawing of any privacy. Partial privacy is no privacy. Our privacy should not be by the grace and favour of any Government. Mediums that ensured privacy in the past have been compromised by advances in technology, so it is only fair that they should be replaced by other secure methods of protecting our thoughts and ideas, as well as information.
I wish you well with your hearing.
Yours most sincerely
name deleted

This is only one story of many posted on Phil Zimmerman's web page.
PGP's most powerful and important use is in the prevention of crime. Certianly it can be used to commit crimes as well, and most like is used in that capacity everyday. But, it can also help to prevent atrocities like those in the Balkans and Rawanda or even, as is popularly suggested, on the scale of the Holocaust.