A lot of big companies make use of popular celebrities for their outdoor bill board marketing. But in my opinion, I believe that it's a limited advertising because not the whole prospective customers would see the bill boards. But when such celebrities are used to promote products on social media platforms, it gives more reach.
Those celebrities are seen as social media influencers because they have a very big fan base that they usually tap into whenever they are promoting a company's product and it's the reason why their services are always sought after by those companies that need their product massively promoted by those celebrities.
I can't afford paying a celebrity to promote my business. Their fee is always too expensive. Some businesses are not making such amounts as their profit. They can never afford paying such high fees for ads.