Why is there a fluid ounce?
Because a fluid ounce and a dry measure ounce (both volume measurements) are about 20% different, though "dry measures" (other than the measuring scoops and spoons in a USA kitchen) have become far less common as most product is now marketed by weight, not volume. 

https://en.wikipedia.org/wiki/Dry_measure

A US dry quart is 1101 cc, while a US liquid quart is 946.4 cc (and an imperial quart is 1137 cc either way, since they evidently didn't have the bifurcation.)

A fluid ounce (volume) of water weighs about an ounce (weight) - a jar that is 10 fl ounces is a volumetric measure and can weigh whatever it likes - more for mercury (or syrup, for a less toxic and more food example) and less for ethanol. The continued use of fl mostly helps to distinguish volume from weight in current labeling, practically speaking.