Why? Because that is how people search.
Think of how you would search for a local mechanic. What search terms would you use? You would probably use a combination of the search terms mechanic, city or zip. This is what the search engines look for.Having a website with content that is only relevant to what you do and not where you do it is a missed opportunity.
Let me give you an example, Pritzer is a firm based in southern California. We cater to clients looking for online marketing in the Inland Empire but we have clients throughout the country who hire us as their marketing experts. Making sure that I mention these facts on this post are very important to potential search results so I added them. You should do the same thing when blogging or simply when updating your website. Make sure to not only mention what you do but also where you do what you do.