Why Automate?

It should be a no-brainer why we automate processes. In fact Continuous Integration and/or Continuous Delivery is NOT possible without automating the repeatable/mundane activities. Humans are not great compared to machines for computing repetitive tasks and performing all permutations and combinations on a certain problem.
That said, a human still has to architect and design the overall system, so should understand what a machine does and more importantly troubleshoot when expected outcomes do not come.

Discussing for the mobile app space, this article shows the costs associated with resubmitting mobile app. The numbers are only suggestive, in fact the costs might be higher than what is displayed in the table. The point is to say that the costs associated with defects/bugs is very dear - lost customers, countless developer hours, brand image and so on.