Answer:
'Time markers' are necessary in some sentences because, without them, the meaning may be unclear or different from the intended meaning.
For example, "Are you leaving?" is the 'present simple' tense, and applies to present time.
But "Are you leaving when you have finished?" or "... at 6 pm?" or "... tomorrow?" etc, is 'present simple with a future time marker', and therefore, although the 'present simple' tense has been used, the sentence refers to future time, not the present time.