For example, a common scenario of needing reference material while doing or creating something:
Project manager writing a budget estimate (excel or email) while repeatedly looking at a project proposal (word or pdf).
Secretary discussing a meeting schedule (email, skype) while looking at calendars of the involved people.
Any newbie learning a complex task or app while having a tutorial video or book open next to it.
Journalist writing an article (CMS, etc) while viewing multiple sources and previous articles.
In many of these cases multiple monitors would be useful as well, but modern monitor sizes easily allow writing/editing a document on one side of monitor while viewing the reference part on the other side. The historical way of doing this is to print out the reference document and attach it next to the monitor - there are tools for this in any office supply store. Now that screen space isn't so expensive, it's more efficient to keep both of these documents in the computer.
If you are a pure consumer of information, you may get by a single window. If you actually do something, then it's different.