As a marketer, you want to ensure that your online content proves useful to readers—answering their questions and providing information they're looking for—and that it also signals to search engines that it's worth featuring high up in search engine results.
So when most of us think about search engine optimization, our mind naturally goes to text first. Optimizing images, if it occurs to us at all, is usually an afterthought.
Today, however, images are prominent part of the online search experience. Think about product images, infographics, how-to processes and workflows, research charts and graphs... and so much more. (Google Images has been around since 2001!)
If we want our content to be found online, it makes as much sense to search-optimize images as it does to optimize text.