I have no clue if using the alt attribute will benefit SEO. Last time I checked, most on-site optimizations added only very little to it, compared to outside linkage. I guess it also depends on the algorithm used by the respective search engine you are asking.
However, using the alt tag is a priority 1 item in the
Provide a text equivalent for every non-text element (e.g., via "alt", "longdesc", or in element content). This includes: images, graphical representations of text (including symbols), image map regions, animations (e.g., animated GIFs), applets and programmatic objects, ASCII art, frames, scripts, images used as list bullets, spacers, graphical buttons, sounds (played with or without user interaction), stand-alone audio files, audio tracks of video, and video. [Priority 1]
and still applies in WCAG 2.0., too.
In addition, the alt attribute is a required attribute in HTML4 and defined as
alt = text [CS]
For user agents that cannot display images, forms, or applets, this attribute specifies alternate text. The language of the alternate text is specified by the lang attribute.
Several non-textual elements (IMG, AREA, APPLET, and INPUT) let authors specify alternate text to serve as content when the element cannot be rendered normally. Specifying alternate text assists users without graphic display terminals, users whose browsers don't support forms, visually impaired users, those who use speech synthesizers, those who have configured their graphical user agents not to display images, etc.
The alt attribute must be specified for the IMG and AREA elements
It's also required in HTML5 and XHTML flavors.