Quite handy to have another pair of eyes sometimes, as was the case when Ryanair stuffed a nacelle in on landing in Dublin, as reported recently.
In this instance the crew were not aware they had touched & the next crew didn't see the damage beneath the engine.
It was stated in the report that the incident was notified by " a member of the public", read "spotter" methinks.
In the past someone was employed to watch every take-off& landing, and normally someone was sat at the holding point checking over departures.
Tower controllers are too busy to give anything other than a cursory glance, & the man in the land-rover has all but disappeared, therefore a sharp pair of eyes are always welcome.