Sometimes cells need additional information to explain how they are used. Of course you can write this text as a label in another spreadsheet cell, but a better solution is to use comments.
Comments are related to one spreadsheet cell only and are only shown if you hover the mouse over this cell. This is a great way to document information that you do not need to see all the time, which keeps your data model clean.
A good case to use this is when you are building a dashboard with multiple layers in combination with dynamic visibility. You might use numbers to identify each layer and use a cell to contain the number of the active layer. You can use a comment to describe the meaning of these cell values.
A little remark about the usage of comments is that they do increase the size of the Dashboard Design file a bit.