If your metadata exists in numerous places, you will spend your development life flipping from one of those locations to another, as you attempt to coordinate actions in your application. Field validation in a field constraint; Table validation in a table constraint; UI actions attached to the form. The list goes on.
The Metadata Manager consists of a database that contains all of your metadata. All of it. The database by itself is a great help. Having a UI to manage that metadata transforms your application development experience: you can now see what happens when data changes, all in one place.
The Metadata Manager is a Lianja application, running in runtime mode, open while you are developing your application. You can make changes to your metadata, and from within the App Builder signal that metadata to be updated in your application. The work of updating the application is handled by Library routines called by a Lianja add-in.
The Metadata Manager uses the ProSysPlus Schema Updater to update your application tables: this is the same Schema Updater you can use to update your deployed apps.