In the old days of search engine optimisation, the wild, wild, west, alt tags were very important.
They were, and still are, a way of coding descriptions behind the pictures on your site. This was great for the old gun slinging spam merchants, as they could stuff as many keywords into the coding of the page as possible. Because of this the search engines reduced the importance they gave to these tags. It is still good practice to have them, but there is a far more strategic use.
Google is moving towards a blended search result. It is not uncommon to see search results now with Google blogs, news, video and pictures included. So that’s where the strategy comes in. If someone does a Google image search, you must describe the pictures so they will appear. In a service industry this may not be so important, but if you sell a product, this is essential. When someone searches for a product that you sell, you want a picture of it from your website to appear.
Go and do a Google image search and see what pops up!