What did the founder expect would happen when he sold the company. Obviously Microsoft wanted to remake a Wunderlist app to be apart of their Office 365 portfolio.
Twilio/Sendgrid/Mulesoft hopes this becomes the case.
People also need to realize that you could make the argument that the massive behemoth that is AWS is just an API company. They give you simple APIs to perform actions (in their case that might be to spin up a compute node, etc).
In the Sendgrid example, they give you an API to send an email.
So it’s about companies who create services and the customer touch Point is only an API
Why else would they have bought them