A /ppp profile sets various parameters used by PPP-like (PPP, PPPoE, PPTP, L2TP, SSTP, OVPN) clients and servers, but not passwords. Local servers use the passwords specified under /ppp secret, or RADIUS, to authenticate remote clients. See https://wiki.mikrotik.com/wiki/Manual:PPP_AAA
Local client interfaces specify a username and password as part of the interface declaration, e.g. a PPPoE WAN connection under /interface pppoe-client will contain name= and password= parameters.
Generally it is best to leave the default profiles as is, typically these would be used by local client interfaces, and create additional profiles containing additional parameters as required, e.g. a remote address pools and local address for a VPN server.