Some brands use their ambassadors to promote their products in an organic and natural way and some other, like this example by Sacoor Brothers, use them purely as a channel to share their message. Cristiano Ronaldo has more than 16m followers on Instagram and that, on its own, makes him a very powerful marketing platform.
A simple post advertising a product or a brand can easily generate almost 1m engagements and reach 5m people at any given time. Clearly reaching a wider audience than the one a static billboard would reach in the same period of time. Moreover, a large percentage of the people who are following Cristiano on social media are genuinely interested in what he has to say, making the distribution of the message more effective.