If you know the password of the SAP* user for a particular client (Eg: client 210 in your case) then you do not need to modify this parameter and restart the system.
You consider this option ONLY if you do not know the password of the SAP* user for a client and you need to access the client with the default password of the SAP* user which is pass.
In order to achieve that you need to delete the SAP* user from the database and then the system will let you login to the SAP system with the SAP* user and with the default password pass.
Regards
RB