Oh yes, this is very important for SEO purposes.  Google can't read pictues, so they use this description to understand what the  picture is about.

It's important to use your key words and/or mix them up a bit/use synonyms.  this will help you post rank higher and enable people to find you images on search, so you may come up multiple times on the page.

SEO - SEO - SEO.  One of the first things Bob Stewart taught me when I joined.

This helps you reference it if you are searching later for that topic.  So if I label a photo city park Clarksville and search those words later to find the photo, that post will come up in the search and help me locate it. It may also give you a little Google boost if you use keywords that relate to your post. 

People can place their mouse over the picture and the description will come up.  If it's a lake in your area people might just want to know what lake.

I was going to say something similar to Debbie R's response. Really I was. I know the description can help with making the post more findable on google. 

So glad you asked John. I hadn't any idea previous to reading through the answers.

Excellent question, John --- as usual Debbie Gartner provides an excellent answer.  

John Meussner It taggs the image for SEO purposes.

Debbie Gartner provided an excellent answer for you.

That helps the search engines index your pohot should someone be looking for something like it.

Good morning John. Looks like you already got the answer, good question.

Google image searchability since images don't "say" anything, Google woulnd't know one from another.

it also gets the photo found on a Google images search... which can be VERY beneficial.