Why this add-on?
Blender allows us to save a .blend file to use on startup.
Also, Blender has it's Application Templates.
While this 2 feature are great, they might not be as flexible or simple as you need...
File > Defaults > Save Startup File, only allows to replace the General template.
Application Templates, on the other hand is more complex to setup, than someone might need.
With Custom Templates it's simple!
Save your template to .blend file, and define it in the add-on preferences!
Customize workspaces, preset your usual materials, define additional windows or anything else you would like to have from the beginning.
The only limit is your immagination!