The origin of the "French manicure" can be found on the catwalks in Paris (France).
Since nail decorations on the French models could possibly divert attention from the "haute-couture" clothing, the designers decided that the models could not have any flashy nailcolours or nail designs.

In order to appear on the catwalk with decent nails, the nails of the models were treated on the basis of natural colors: White nail-edges combined with a natural or rose colour for the nail bed.
The French manicure was an immediate success on the catwalks and became an instant trend in the whole world.
Nowadays this form of nail decoration has become a standard in naildecoration.

French manicure is on top of that extremely suited for camouflaging the difference between shorter and longer nails. The white edge gives the impression that the short(er) nails are (optically) longer.