Before electronic amplification, instrument makers and musicians had to find newer and better ways to make themselves heard among ensembles and orchestras and above the din of crowds.
Many of the acoustic instruments we're familiar with today---guitars, cellos, violas, etc.---are the result of hundreds of years of experimentation into solving just that problem.
These hollow wooden resonance chambers amplify the sound of the strings, but that sound must escape, hence the circular sound hole under the strings of an acoustic guitar and the f-holes on either side of a violin.
I've often wondered about this particular shape and assumed it was simply an affected holdover from the Renaissance.
While it's true f-holes date from the Renaissance, they are much more than ornamental; their design-whether arrived at by accident or by conscious intent-has had remarkable staying power for very good reason.