Opening an application while pressing the shift key

Recently, Microsoft Word 2011 failed to open and showed the message, "Microsoft Word has encountered a problem and needs to close." I followed troubleshooting instructions on the Microsoft website. Nothing cured the problem, but creating a new user account did enable Word 2011 to open normally. That seemed to imply that "a corrupt user account" was causing the problem. I could not find any instructions about how to repair that, but at some point I came across the suggestion to "hold the shift key down and click on the Word icon in the dock." I did this and Word opened normally in my account. After this, Word opened normally without holding down the shift key. Apparently this fixed the problem.

My question is, what does pressing the shift key do while you open an application? What's different? Why did it work? What's special about the shift key in this situation?

Thanks.