Why are the tropics of cancer and Capricorn important?

Answer 1

The Tropic of Cancer is a line of latitude (23.5 o N) where the Sun is directly overhead at the Summer Solstice.

The Tropic of Capricorn is a line of latitude (23.5 o S) where the Sun is directly overhead at the Winter Solstice.

Answer 2

They are the farthest possible north and south, respectively, where the sun can be directly overhead at a point on earth.