By default when you create a new Phoenix app using phx.new, a set of files and configurations will be generated for Brunch. Though the Phoenix team decided to use Brunch, you don't have to. You may not want Phoenix to handle asset building or you may just prefer another tool. Either way, if you'd like to opt out, you can include the --no-brunch flag when generating the project.

$ mix phx.new --no-brunch my_app

If you have an existing project that you'd like to remove Brunch from, there is some information in Phoenix's Static Assets documentation.