The reasons why I want api for my consumer apps have more to do with my own automation desires than with any personal assistant.
Our kids use a kid friendly debit card product where we can fund their accounts on demand. I also have a chore schedule system I built where they enter their chores each day. I'd love to be able to automate allowance based on chore completion but there's no way to integrate the two.
Every 2 weeks I enter all my accounts, credit cards, etc into a spreadsheet for graphing and tracking. I'd love if there was an api for these services to fetch the data instead of manually logging in.