It has more to do with tradition, once assigned you do not change symbols like these.
Also, it is just one of the many insignia that mark an aircraft as belonging to particular country.
There are other ways to identify an aircraft like
1, The roundel.
A roundel in heraldry is a disc; the term is also commonly used to refer to a type of national insignia used on military aircraft, generally circular in shape and usually comprising concentric rings of different colours.
2. Bort Number--
Something like the registration no. of vehicals. Differant countries follow different systems and hence the info this no. conveys varies from country to country.
You can see the no. "SB 115" just above the fin flash in this pic of Su 30 mki.