When do we give a 'heads up' to someone? Why?

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When we wish to inform someone about the details of a particular theme, topic, item or person - I'll give you the heads up about the new policy. The idiom appears to have a military origin - a heads up indicating that an important announcement was pending.
An early citation for the contemporary use of 'heads up from 1977
Early use in the late 1970s stressed the importance of the information supplied:
"In a message characterized as a 'heads up alert', intelligence officials warned ... that Arab diplomats had suggested that Ambassador Andrew Young meet with a Palestine Liberation Organization official." The Washington Post, August 1979
In recent years, however, 'to give a heads-up' has become synonymous with the less glamorous 'inform'

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