Historical convention. People had to have a way to define that a certain key combination had to be pressed / had to be informed that a certain combination had been pressed. Since a single caret is rather unlikely to occur in regular text it stands out.
VIm is the descendant of an editor from that era: vi. Without a GUI (no pointy-clicky) all input had to be done via the keyboard, and as such key combinations had to be used. gvim is just a fancy interface for VIm, and as such supports all the same combinations. Documentation on VIm can be found
here, or by entering :help when in command mode.
