Following that tutorial, if it’s up to date enough to follow will set you up with an environment for working on Sponge itself.
This is useful for plugin development, as it helps to find why you see certain behaviour and explore the minecraft source as well as making PR’s to Sponge. But it’s by no means required.
SpongeStart Gradle plugin is a plugin that someone made in order to run a server from a dev environment without having to keep a Sponge dev environment up to date.
I recommend using Minecraft Dev for IntelliJ however if you are using IntelliJ it makes things a ton easier.