There's really no significance other than for your own business needs. And as you say, it just groups them and uses a different URL. We have used them in the past for grouping together different types of pages or segments, but for the most part we usually stick to a single microsite in our instance.
Do we have any reports based on microsites?
There are reports in Oracle BI (Business Intelligence), or what is being referred to as new "Insight" which is in a controlled release state. However, I will be the first to admit, I don't typically rely on landing page statistics from Insight/Oracle BI, I prefer tagging with 3rd party analytics for viewing landing page performance.