If portions of your website are not reasonably accesible to software which allows people with visual limitations to navgate the internet, you could be involved in the next wave of ADA/accessibility lawsuits. Especially if you offer "internet only" promotions, if people with visual limitations can't access the products and services you provide, your website could be a lawsuit waiting to happen.
Recently, a major website reached a settlement with the U.S. Department of Justice which would require it to ensure that content complied with the Web Content Accessibility Guidelines 2.0 Level A and AA Success Criteria ("WCAG 2.0 AA"). Because other major websites have settled similar claims readers should consult qualified counsel and experts to ensure that their websites meet applicable standards for accessibility by those with disabilities and/or visual limitations. More information about the WCAG 2.0 AA standard is available at: http://www.w3.org/TR/WCAG20/.
Programs like "JAWS for Windows" and other software can read out text on a web page, but only if that text exits. For example, if there is only a picture of a product and a price (with no written description of what is shown in the picture), a person with visual limitations might not be able to determine what is actually being sold for that price. But if a description of the product is posted next to the price (in standard HTML text), then the program can probably read the description, which is hopefully complete enough to allow the visitor to determine if s/he wants to purchase the product.