Indie rock (in the broadest of senses) lives and dies on credibility, so much so that ‘indie cred’ is a common phrase. Sure image helps and gimmicks help and music/ songwriting ability may also help, but cred is important. I won’t go as far as saying that cred is king, but it’s certainly important.
And yet, indie rockers often take on genres they are not intimately familiar with. That is, they lack imediate credibility. Beirut takes on Balkan brass music. Iron & Wine take on southern folk.
Even when indie rockers are not taking on other genres, they are often tackling music that isn’t familiar to the listener. Sufjan’s epic folk, Joanna Newsome’s weird harp screeching, Andrew Bird’s experimental, looped violin pop and Neutral Milk Hotel’s emotive fuzz pop are all examples.
What do these people have in common? Unique voices. And they don’t lack credibility.
The unique voices lead to the listener to view these artists with more authenticity than otherwise. If they not going to sing normal then they must mean it.
I’m not sure this is a conscious thought on the artists part, but it’s more likely some bred by the culture.