What is Procfile? and Web and Worker

Procfile is a mechanism for declaring what commands are run by your application’s dynos on the Heroku platform.

From Process Types and the Procfile, which is a good introduction, but basically you use the Procfile to tell Heroku how to run various pieces of your app. The part to the left of the colon on each line is the process type; the part on the right is the command to run to start that process.

Process types can be anything, although web is special, as Heroku will route HTTP requests to processes started with the web name. Other processes, such as background workers, can be named anything, and you can use the Heroku toolbelt to start or stop those processes by referring to its name.

So, in short, worker is not necessary, unless you want to run some other process in the background by controlling process with the heroku ps command.