Heroku is a great product if you want development speed right from the start. I have used them in the past, and it was a very nice experience overall.

For many projects this is more than enough. However, in my case I would be paying 2-3x more if I was using Heroku.

With Kubernetes, in case I wanted to deploy new projects or even spin up a "testing" env, I can use the same stack/cluster, and not have my costs increase.

I do realize that this is not for everyone. But for me it just made sense, and enabled me to work on new features faster than before.