So what is an aPaaS?

This is where it gets exciting.
With a PaaS, you have to define, build and implement the way the code you have written is deployed onto the Cloud Infrastructure but your Runtime environment, any required middleware and the Operating System are all provided and maintained for you.
But some people / some companies “just want to code” and don’t want to have to worry about anything beyond the design and development of their application, and some companies want to be able to choose not to have to worry about it. They like the idea of being able to deploy their software at the click of a button and pay for someone else to do the rest.
Which is where the aPaaS comes in; it stands for Application Platform As A Service and it’s a cloud service that provides everything a PaaS does, but it also provides the deployment environment too:
PaaS: development and deployment is your responsibility
aPaaS: only development is your responsibility
Note that an aPaaS is really only relevant if at least some of the IT Service you want to use or provide involves building the software yourself.
Just how exciting is an aPaaS?
As it turns out, very. An aPaaS is perfect for a start-up that has an idea that they want to develop and deploy and get feedback from their target audience really quickly, or perhaps they don’t already have the deployment expertise and can’t afford the cost of employing a separate DevOps Engineer to manage the deployment, or it might even be the case that although they can afford that extra pair of hands, if given the choice they’d rather use the money to employ another developer who can design and develop more revenue generating software IP.

Source: https://martinjeeblog.com/2017/04/23/what-is-an-apaas-or-how-not-to-make-a-paas-out-of-you-and-me/