King Edward VII of England, by Sir Samuel Fildes, 1902, Oil on Canvas
In this example, the velvet drapery and column are used to symbolize the power of the King, relying on a convention established much earlier and legitimized through subsequent royal portraits, such as the one below.
King Willem III of Netherlands, Nicolaaas Pienemann, 1856, Oil on Canvas
Using convention allows artists to relate their image to the images already within the public consciousness, tapping in to the symbolism and meaning of those images. The application of extant conventions to new uses in some ways constitutes the evolution of imagery. It is like using a given formula (f(x)=2(a)+3(b); a=2,b=6) but changing some of the variables (f(x)=2(a)+3(b); a=1,b=4).
It is the same process for any image, a royal portrait or an advertisement. In fact, because of the legitimizing effect of convention, images made for popular consumption may rely more heavily on them.