Google’s John Mueller clarified in yesterday’s webmaster hangout video that while Google does look and read the alternative image attribute (alt tag) for use on web search, it plays a small role there. It is more heavily used by Google in image search and that is the primary SEO reason to use alt image text, not for web search but for image search.

John said this at the 9:40 minute mark into the video:

Here is the transcript:

So for web search – it is used a little bit.

Forum discussion at Google+.