Much of the reason for that is the type of society seen during the Renaissance period, and the way in which it used art to adjust how future generations would view its leaders.
With the lack of photography for several centuries, how would the rulers of the time be remembered? The answer was through portrait paintings and equestrian compositions allowed an extra element of power and strength to be added. The image of a strong leader, sat upon a powerful beast, that he controls completely, will always attact the viewer's interest and instill an impression of how they were in their own time.
This drawing from Albrecht Durer does not identify the particular knight, meaning the work was more for artistic value than delivering a valuable commission. There is a small possibility that it was a study piece for a larger painting, as equestrian forms are hard to achieve without considerable practice and study. Even the great masters that produced equestrian portraits, with or without accompanying riders, such as Rubens and Velazquez went to great lengths in order to develop their craft.