Some of the cool things SF will automatically take care of when it comes to sharing tables

 1. If you change owner - all manual shares with Row Cause as Manual will be truncated .

2. If you insert duplicate share records - salesforce will automatically do the duplicate management 

3. If you change the account on the contact associated to the community user all manual shares of the user will be truncated

If you do not want the manual shares to be truncated ( when owner changes / community user’s account changes ) use custom Apex sharing reason when inserting the share records.