Use 'on earth' to stress your amazement or disbelief in a situation or fact.

Expressing Astonishment

'On earth' can make questions seem rhetorical, implying no answer is expected.

Rhetorical Effect

Overusing 'on earth' can diminish its impact. Use it sparingly for emphasis.

Indicate a product or phenomenon is found or recognized all around the globe.

Sometimes 'on earth' is used figuratively and does not literally mean every place on the planet.

'On earth' can be used for hyperbole, to express that something is very common or widespread.