For a user to gain access to a linked server, the linked server must validate the user for security reasons. The requesting server (that is, the local server) provides a login name and password to the linked server on behalf of the local server user. For this to work, you need to map the local logins with the linked server logins you are going to use. Remember that
sp_addlinkedserver creates a default self-mapping for all local logins to the linked server. You use
sp_addlinkedsrvlogin to specifically control the logins that you want to use the linked server.