I think "Display advertisement” is not a use case of its own but a Cross-cutting_concern
that can be attached to many different use-cases:
a use case is a list of steps,
typically defining interactions between a role [...] and a system, to achieve a goal.
the end-user does not visit a car-webportal to see advertisments but to achive a goal: i.e. find infos about a certain type of car.
Showing advertisments is a Cross-cutting feature.
The goal of the car-webportal is
- to attract car-entusiasts
- and to make money by selling car-related ads.
- the more car-entusiasts visiting the website the more money they make.