The Welcome screen's primary purpose is to provide new users a quick, four-step process that walks you through creating a new report through the Report Design Wizard. This is the default view when you start Report Designer, but if you close it, you can make it reappear at any time by going to the Help menu and selecting Welcome.
In addition to the new report creation buttons, the Welcome screen also shows a list of sample reports. You might find these useful if you're looking for inspiration, or if you can't figure out how to use a certain Report Designer feature. In order to display the samples, you must have the Pentaho sample data HSQLDB database installed and running.
If you do not want to see the Welcome screen at start up, you can uncheck the Show at startup option in the lower right corner of the window.