Well the same reason less than ethical webmasters use very near color CSS to hide text. They want or need the text there, but don’t really want you to see it. Webmasters however, are penalized for this deceptive technique, their rankings damaged and in turn their business damaged.
Hidden text can occur for many reasons, most are in no way malicious. Text can be same color text and background, hidden in html commenting, stuffed in to alts and titles and hidden with numerous CSS styles. All of the common findings of hidden text, malicious or not are a violation of Google’s quality guidelines…