First off, what I notice about all the liberator heroes is they have a proud nature to their characters. George is standing upright with a sword; Marat, thought injured, is smiling, as showing his sense of pride and accomplishment of what he's done; Toussaint is in uniform with sword in hand, as if gloating of his recent victory; and finally Bolivar is in uniform, in a formal pose, sword in hand, as if he just won a great victory as well.
The heroes are portrayed as proud to show future generations the heroism behind the men who liberated their countries from previous oppressors, and to help to carry on that pride.
These revolutions needed Heroic Figures as front runners, because they were the driving force behind the revolutions themselves. They were the ones who reminded all the revolutionaries what they were fighting for. George Washington had to encourage his troops through 7 desperately cold winters, otherwise the British might have retaken the colonies. Marat had to encourage the people of France to topple a few century long-standing empire, which could not have been done without inspiration. Toussaint had to encourage Africans, one of the most put-down races in history, to rise up and defeat their white owners. And finally Bolivar was faced with the difficult task of creating a rebellion in one of the newest and most uninhabitable places on earth, and continually had to encourage them! This is why heroes are needed: so humans remember the cause they're fighting for.