You are important, because you are one of a kind,

 There is no replica or clone, just you.

Without you, people, nations, worlds, would be lost.

From that day you were born to the day you die,

Everywhere you have touched is affected.

Without you streets, buildings, cities, would change.

Who− you, What− are important, When− from birth,

Where− everywhere,, Why− because you are one of a kind.