OzzyOzBorn
It may not be as important to advertise the airline, but its still important to advertise the destination, especially when it comes to tourist destinations. A lot of people pick their holiday destinations based on whats available from the local airport - without advertising they'll automatically go to Tenerife or Paris/Amsterdam for city destinations. But if you launch a campaign and tell them that they can also go to Gdansk and Budapest etc. then some people will decide to try those destinations for a change. Additionally, some might assume they cant get to those destinations from their local airports, and so search from nearby larger airports instead without even considering it.
Advertising is still crucial for the success of a route.
Even when it comes to brand awareness, it's still very much a thing. Given the choice to fly between Ryanair and Some-airline-you've-never-heard of, guess which wins? Even if Ryanair are slightly more expensive, people will pay the extra rather than risk an airline they know nothing about. A lot of people are nervous fliers, and just dont want to risk an unknown airline.