These username/password pairs have several uses such as PPP CHAP authentication or local access authentication if you enable aaa. The 'default' is a keyword which specifies all interfaces who have not had their configuration changed and therefore this will apply to all user access methods, console, telnet etc.
the http server is slightly different to this and has its own config. This can the either set the authentication method to local, to user the username and password pairs, or to aaa to use the config set for aaa. I suppose it just depends on your view on access to the platform as to which you choose.
aaa new-model
aaa authentication login default local
username fred password bloggs
ip http server enable
ip http authentication local
or
ip http authentication aaa.