The philosophy behind Backstage is simple: Don’t expose your engineers to the full complexity of your infrastructure tooling. Engineers should be shipping code — not figuring out a whole new toolset every time they want to implement the basics. Backstage allows you add “stuff” (tooling, services, features, etc.) by adding a plugin, instead of building a new tool. This saves you work and avoids the need of your team to learn how to use and support yet another tool.
Read all about how Spotify uses this internally in this blog post.