Let's say you have a web site that just serves audio or video or images or other forms of multimedia - will Google be able to understand it all? John Mueller of Google said on Reddit "You always need textual content on-page, regardless of what other kinds of content you might have."

In short, Google needs text to better understand what your audio file is about, or what your video file is about and so on. It isn't able to fully understand all the information in those files, yet. John wrote on Reddit:

You always need textual content on-page, regardless of what other kinds of content you might have. If you're a video-hosting site, you still need things like titles, headings, text, links, etc. The same goes for audio-hosting sites. Make it easy for search engines to understand your content & how it's relevant to users, and they'll be able to send you relevant traffic. If you make it hard for search engines to figure out what your pages are about, it would be normal for them to struggle to figure out how your site is relevant for users.

Although, Google is getting way better at understanding all sorts of multimedia files these days, you still can help Google by giving it text.

Forum discussion at Reddit.