I’ve been to countless concerts, seriously well over 100 shows, so hopefully my perspective will help some. The biggest thing, I think is that songs that people know from concerts are tried and true classics that most fans of that band–or at least the genre if they don’t know them and are there to see a nother band–can enjoy. With new songs, it’s often hit and miss because the band or label may think it will do well, but it’s not really until they see the fans react to or ask for a song that they know how it will be received. That said, sometimes a new one is dead on, and it’s fantastic.
Also, the main reason I go to concerts is to really relish in the energy and experience of the music. Part of that is being able to sing along or know which parts of the song have epic solos, lyrics, breakdowns, whatever. Seriously, being able to sing along to a favorite song in unison with the crown is the sort of experience that gives me chills. This simply isn’t possible with new songs unless it’s particularly catchy and predictable which, at least for me, makes it difficult to enjoy because I’m not a fan of those things.
Concerts are noisy. Between so-so mixing, crowd noise, parts lost in the live translation and all, sometimes even a great song is difficult to really follow effectively. With a song that I already know, even when it gets muddled by a bad sound crew, I can still follow it and enjoy it. If it’s a piece I don’t know, I can get lost and confused when that happens.
Finally, if I’m there to see the band, I like that band because of certain songs; I want to hear those songs. If I’m going to pick out a handful of songs that I hear more often than others anyway, why wouldn’t I want to hear those songs live? In some cases, seeing a live version can even help add to my appreciation because I can see which parts are perhaps more difficult or are really meaningful to the band. As an example, seeing the vocalist close his eyes and really focus on the words rather than doing all the stage movement, or seeing the guitarist lay his head back during a passionate solo or similarly seeing his crazy finger movement on a complex one… it adds a whole new level of depth to the piece for me and can make a song I already love even better.