Nato intercepts – Why?

Why do NATO aircraft intercept Russian aircraft? If they are no threat, and you know what they are, why intercept them?

Is it purely to help them navigate through civilian airspace where airtraffic is heavy? And if so, why can’t that be done via radio? Does the interception make any difference to the flightpath of the Russian aircraft?

Is it all built up tension because of the Ukraine and others wanting to join Nato?

Jay
UK