Starting with Office 2010, Microsoft put a minimize and close button on the upper right corner of the splash screens of the various Office apps. But, the splash screen is up for such a short time, what is the reason for doing this?
The Microsoft Office suite appears to have put a greater emphasis on Add-Ins. For default installations, none are included hence the splash screen only stays up for a very short time.
However, for Office power users, they may have many Add-Ins (especially enterprise users who develop their own custom AddIns) which will significantly slow the startup time when the plug-ins run.
AddIns are not controlled or regulated by MS so they may load for a very long time running code, performing updates etc.
Here's what my Excel with 1 Add-in looks like:
The close and minimise button was probably added to enhance the UX, to allow the user to close Office should they accidentally open it, or let their Add-ins run perform updates in the background etc.
If your computer is slow/the application is taking too long to load, you have the option to minimize or close the launch of the application. It is all about making the user feel in control at all times. Most of the time the user has no control if he launches the application, look at adobe suit, the applications take a while to load and there is nothing you can do to close them if you launched them accidentally. The splash screen controls help you bypass this problem.