I think the only time you should use both artist and songname is if your after a particular song and multiple artists have a song with a similiar title or a cover version of the song, you are looking for. If you can't find the song via
artist or song title, then try album, and if your after a particular genre of course use "Genre" drop-down menu. But what happens if you can't find the song, or artist, or album??

There is more than one answer, maybe no one has ripped the songs, if so you or somone you know has a good copy on cd you could rip it yourself which would be the common sense thing to do. The other reason normally is to do with very rare releases, deleted items and of course import releases like an album might be available in Australia but hard to get in the US or UK and vice versa. And of course the final reason it's just not on cd, maybe 45'' or LP.

If you're after an a specific release but can't find anything...best advice:"do a google search", you find alot of information on some releases through the google search engine. Actually a number of good bands have certain releases which only have 500 or so copies worldwide - which means that the songs from that release are harder to find than your average easy to find releases. (the more popular the band the easier to find files & releases)