Expression "What on earth for"

[Thillaiganesh]

Mike: They went on strike last week

Jane: What on earth for = Could you please let me know what is the meaning of this sentence

[Gray]

Jane wants to ask -- for what reason they (workers) went on strike.
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First lesson - English, not english. I, not i. ~A student of English

[Cerberus™]

To add "on earth" to a question gives extra emphasis to the question: it means that you are very surprised at the thing you are asking about.

[Anna.ha]

Some similarity is you can say to yourself that "who on earth could that be" when someone is knocking the door of your house or department.

[Cerberus™]

Yes, that is the same.