You might want to shine a light while you’re observing – perhaps to adjust your telescope or check a star chart.
But the wrong sort of light can dramatically affect your observing. A normal white torch will be so dazzling that your eyes have no chance of observing faint objects.
The answer is to use red light. This allows you to see while having no effect on your eyes’ sensitivity to the sky – what astronomers call your dark adaption.
Avoiding white light is especially important if you are observing with others – at a star party, for example. Blinding others with white light will not do a lot for your popularity.