The companies with most to gain from outdoor advertising are physical stores, restaurants, etc. -- places where people do spontaneously go but whose primary difficulty is literally getting people in the door. A billboard or other kind of display can alert drivers (for instance) to a new place to stop in at. Signs tend to be remembered in a local context as well -- where it was seen ... where you were on your way to ... -- which helps remind people where the business is when they decide to look in later on.

The same applies to upcoming events, festivals, etc., which benefit from being remembered in the context of a locale.

Freeway billboards are notoriously low-converting. Yet they can be effective as well. By way of example, I saw one the other day beside the freeway for tattoo removal. It was an excellent ad -- very effective in raising the topic. Unfortunately, I have no tattoos to remove; so it was wasted on me. Worse still, they were pitching a forgettable name / URL. So even if I had a tattoo to remove and were now determined to pay for the service, I'd now end up going to Google -- where, the odds are, I'd find a different local company. Essentially, the company behind the billboard was advertising their niche (including their competitors) and not themselves.