In Marketing Cloud, the term "business unit" is a bit confusing, so let's begin by defining it. A business unit is a shared workspace. Anyone with access to it can see and edit the data and content inside it.
A business unit doesn't have to map precisely to the unit at IU you work in. Many IU units can share a business unit—they do not need to be 1:1.
Situations when you should request a new business unit
If the data you'll be putting into Marketing Cloud is sensitive in nature, such as patient data, highly regulated research participant data, or anything else that needs tightly controlled access, you should request a new business unit rather than share with others.
Situations when you should share a business unit
If your unit is one of many that reports to a larger organization that works together on similar efforts, it's highly likely that sharing a business unit will be the best approach. It allows for redundancy. For example in IU Studios, the people who produce all presidential communications, IU Today, and JagNews all share one business unit. Several different people share one business unit.