A blog can be a great addition to a website for many reasons. A blog may allow you to add useful information to your website without cluttering up your main navigation or home page and can also help with search engine optimisation.
Adding fresh content to your site looks good in the eyes of the search engines as they see your site as active and regularly updated but also you may find that many of your blog posts appear in the search results when someone carries out a search relating to the title of your article.
The cost of adding a blog varies depending on your website and the content management system you use. Some just require a plug in or additional page creating where as others may need to be written in from scratch.
Once you have a blog on your website, be sure to keep it regularly updated by adding new content to it a few times a week or at least monthly to really get the benefit from it.