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It's not that hard a concept, let's use that part of property tax that goes to say, road maintenance.

You drive a car on the road,  

...your neighbor could drive several more than you do, thus deriving more benefit for the same flat tax.

This same analogy plays out across the spectrum.  Taxes should be progressive to account for the varying levels of benefits received.

A man with a house in one part of town is not benefitting from the same fire protection offered to another in a different part of town.

Downtown core business benefit from more police presence than outlying businesses.  Should they pay the same rate?

Flat taxes fall most severely on those who are least able to shoulder the additional and unfair burden.

Taxes should be progressive to reflect the varying degree of benefit derived from said tax.