William McGurn Explains the Logic of a $410 Billion Omnibus Bill Full of Earmarks - WSJ.com

This is the best explanation I've seen.
It doesn't necessarily make them good, or right!...

William McGurn writes in the Wall Street Journal:

"The answer is that politicians and citizens understand earmarks in different ways. Politicians understand that not all earmarks are pork, and not all pork comes in the form of an earmark. They also appreciate the ease of inserting pet projects into large spending bills without any debate or scrutiny.

The public understands that this way of handling taxpayer dollars is corrupting even when it doesn't lead to a federal indictment. In fact, since the Jack Abramoff scandals and the now-notorious 'bridge to nowhere,' earmark has become a dirty word. So when citizens see these earmarks exposed in the press and lampooned on late-night TV, they assume it makes it more difficult for our congresspersons to pass them.

What the public does not understand is that the more earmarks there are in a bill, the harder it will be to vote against it. The reason is simple: With every earmark, a congressman or senator gains a personal stake in the passage of a bill he or she might otherwise oppose.

Which brings us to the real scandal here -- that 8,500-plus earmarks adding up to $8 billion will end up sticking the American taxpayer with a $410 billion spending bill that is filled with large and significant provisions that have gone largely undebated."