Basically a mass in Revit is something "completely" different form the normal revit families (like furniture).
As you can see it is one of the things that has a separate entry in Revit, and important it is called a conceptual mass.
Conceptual Masses, as the name implies, are more for conceptual design (think buildings, group of buildings).
You can further see the difference that the mass has its own sets of tools and that even the environment looks different, it is often referred to as the Conceptual Design Environment (CDE).
It is most times used for quick design studies. As the design develops Revit has the ability to convert those masses into walls, floors, roofs etc.
Masses can be used for other purposes of course but that is the main use/purpose.
All that said it is perfectly fine to create furniture in the CDE.
This often the case in Revit. There is no fixed way to solve things. Some ways might work better in your situation or fits better in youir workflow.
I usually do no concept studies and have limited use for organic shapes, therefore do not use the Mass tools that often.
However my advice would be to leave the the mass tools for some time later as it might be confusing in the beginning.
Louis

Please mention Revit version, especially when uploading Revit files.