Annotating other websites was a side quest along the way for Genius, but its original creation was focused on annotating rap lyrics. It took them 5 years to drop "Rap" from the name "Rap Genius". https://en.wikipedia.org/wiki/Genius_(company)#History
Though Wikipedia makes it sound like there was a lot of pressure from investors to pivot into that side quest:
In 2012, the company received an additional $15 million investment from Silicon Valley–based venture capital firm Andreessen Horowitz (also known as a16z),[1][3][11] prompted in part by partner Marc Andreessen's own past effort to build a group annotation feature into a web browser.[1] Ben Horowitz described Genius as "one of the most important things we've ever funded"
