A Default view is a standard map view which is listed on Project page for all project members.
In opposite Personal view is a map view which is listen only for its creator and is hidden from other project members. However they still can access it by its link.
Your Personal saved views are marked with a bookmark icon an a project list. Indicating only you can see it. Other views are considered as Default views.
Use Personal views to :
Reliably Save state of a map view.
Share it with your colleagues. (Consider Share feature instead, continue reading)
To build Stories in Stories Builder.
All of it without polluting project for all members.
In opposite to Share feature Personal views are reliable. If you need to persist state reliably for a long time, use Personal views. If you need to just quickly and temporarily share something to your colleagues use Share links.
See documentation to understand how it is stored in project metadata.
See how to use Personal view to Save current state of a view.