Heroku gives you servers, but manages them for you - so you have long running processes, for web or worker etc. Lambda allows you to have the process run for literally the lifecycle of the job (request, streaming job etc.). In fact, your job can't run for more than a few seconds or it is terminated. Heroku charges you for the hours your dynos run - but what if they're idle most of the time? You still pay for them - not with lambda.