Why do the Tropics of Cancer and Capricorn exist?

Answer 1

They only exist because people mark them on a globe.

But they represent the most northerly position or most southerly position at which the Sun may appear directly overhead at its highest point.

This event occurs once a year, at the time of the summer solstice - in June, northern hemisphere - or - in December, Southern Hemisphere.

Answer 2

They exist to define the tropics.