The single largest source of Bandcamp sales

Search.

More specifically, lyrics!

As stated in Bandcamp’s support site, search is driving the traffic when it comes to purchases of music on the platform.

Google searches are the single largest source of sales on Bandcamp (surpassing even Facebook), and your fans aren’t just searching for the name of your band, or your latest track — they’re frequently searching for that one lyric snippet they remember.
~ Bandcamp Help

This is why it’s important to make sure your SEO is dialled in, your website is optimised and that your lyrics are available for search engines on sites like Bandcamp and Musixmatch.