There's a sweet spot of early startup or side project where you don't have the time, budget or people to manually set up and maintain servers on your own or deal with the complexity and cost of Kubernetes or AWS, especially when your focus is on building the product and acquiring customers.
Heroku (before its inevitable enshittification under Salesforce) was great for this use case. Sure you will outgrow it at some point, and it did get expensive, but when you just want to throw up an MVP with minimum fuss and maintenance you could do much worse.