I would wager that 90% of the business for Plaid, Yodlee, and Intuit is account verification; the thing that you used to do by having small ACH transfers of random amounts that you verify. The fact is that 90% of running a fintech business is identifying and bounding fraud risk, and these "banking API" companies are able to move the needle down a couple of basis points.

edit It's shit like this that just screams for the Fed to force FIs to implement a standard API for verifying accounts and making transfers. I bet half of fintech would collapse overnight, but the collective cost savings would be in the billions.