The standard tier at Heroku is 512MB RAM for $25. Over at Vultr, I can get 2GB RAM for $20. I also get 45GB of SSD, so I don't have to pay for something like Amazon S3 to store uploaded files.
If you're building an actual product, I agree with you, pay Heroku and focus on building your product. However, there's a lot of people looking to host smaller projects that wouldn't break even financially if they used Heroku.
There are lots of reasons why users need a different, or especially an open source, PaaS.
Some users run into scaling problems when their products grow beyond a certain point. Others want to have more control over their infrastructure for compliance, governance, or other administrative reasons. Others have huge deployments and want to save money by using their own cloud accounts.
It varies from customer to customer, but for many Heroku isn't an option or isn't the best option.
This looks like a nice middle ground for me. I should be able to migrate to flynn on Digital Ocean pretty easily (easy deployment of flynn infrastructure, same buildpack system). I'm sure there'll be regular maintenance involved, but it looks like nowhere near as much as a roll your own kubernetes cluster.