It's all about having a common denominator for adding & subtracting. And, like salukipv1 said, it's probably easier for some peope to "see" the difference on paper between 1/2 & 5/8.

And the gas prices...I argue with people about this all the time. lol Almost everybody I know will see a price at say, $3.09 & 9/10 and say gas is $3.09. No, in my mind it's $3.10. I think it's just a marketing ploy...I could be wrong though (I have been before). lol