Typically users want to be able to identify those records within Workbooks for which they are responsible (either permanently or temporarily as part of a business process).  This can be achieved using the Assigned to field.  Any records assigned to a specific user will appear in that user's 'My' view on the relevant Landing Page. 

In addition, there may be some records that aren't assigned to you but that you want to 'track' and identify easily.  Activating the Watch functionality on such a record will add that record to your 'My' view on the relevant Landing Page.

For example, a Sales Manager might want to Watch an Opportunity that's assigned to a member of their team and have it appear in their 'My Opportunities' view, which they can achieve by activating Watch .  Similarly, you can stop watching a record, which will remove it from the 'My' view.