Why use SSL in the APIs calls?

SSL is a protocol of encrypted information exchange between the customer and the server, and it becomes mandatory when it is about sensitive information.
When you use a digital certificate, you ensure that all the exchanged data is visible only to the client and the server.
Regarding the information exchanged through APIs, they are mostly confidential, and must not be visible to third parties. Only the stakeholders must know about the content of the exchanged messages.
This type of behavior may be obtained through data encryption before the information exchange starts.
Besides that, if you have an online store and want people to find it, did you know that, since 2014, Google prioritizes in its organic search results the websites with digital certificates?