Wherein I Actually Do Not Complain About George W. Bush

I read this story on CNN about the latest pardons the President granted and it made me wonder how you get a pardon from the President in the first place. It doesn't particularly matter to me that Bush has granted so few. I just want to know how any President decides what pardons to give. It seems to me that offenses that happened over 20 years ago, for which the offenders received probation and fines, wouldn't even be worth noticing, let alone worth noticing by the President of the United States. Why do Presidents give pardons in the first place?