Without Clients, you can not add a Work.
Clients are categorised as Inactive (old Clients for which you have stopped working for) , Current Clients and Future Clients (clients to whom you have given proposal and they are still not accepted).
'Referral by' is the person who referred that client to you. It will help you to know in future who is referring you clients.
Income Tax and GST Passwords are not compulsory for adding a client. You may decide to save that data separately. But also understand that it is totally safe and confidential if you decide to keep this data in PracticeGuru.