Lets you have a performant production server with a couple clicks, no devops knowledge needed.
Has tons of plugins for all your needs: Database (Heroku Postgres), Cache (Redis), Monitorization (NewRelic), emails (Mailgun), SSL…
Provides lots of documentation to set up apps in a variety of languages, and covers things like using several environments, tuning your memory usage, deploying from Dropbox, and more.
Together with a auto-scaling plugin like HireFire, it can manage your app as usage goes up and down, and you'll never be overwhelmed by a traffic spike, or spend more than you need on valley hours.
The pricing is a little high, especially if you want to use the dedicated dynos, which increase performance and decrease response time dramatically. We've also been bit by downtimes a couple times in the past year, being fully managed by them there's not much to do once it goes down. Also, since it runs on AWS ,if that goes down Heroku does too.
Great for starting a web app. As traffic rises, consider switching to a performance dyno, which gives you more bang for your buck, and also has much slower response time. Beware of delays in your requests when you deploy, consider using rolling restarts to fix it.
I can save lots of tiie configuring, testing and deploying servers on AWS or providers like Digital Ocean, while focusing on developing the app itself, which is what brings value after all.