www.Johnchow.com has an excellent post about the importations of deep linking. With the advent of scrapers (A robot that reads blog posts RSS feeds and creates copies of content for there own website) alot of your content can be easily stolen but if you include some links to posts in your post they will be copied to the scrapers website and you should be able to get some [tag]SEO[/tag] benefits.
Another reason I deep link in almost very posts is because of scrapers. I offer a full feed RSS. That makes it very easy for scrapers to rip the entire content of my blog and post it on a scraper blog. Most scrapers rip the entire post with all links intact. If your post has a ton of deep links back to your blog, then you may get some get [tag]SEO[/tag] benefits and visitors from the scraper blog.
The post also goes though a few other reasons that deep linking in posts is important.
When writing a blog post you should always try to deep link to some of your older articles. You cannot control what others link to on your blog, but you can control what you link to. By deep linking to your older posts, you help your readers discover posts they may have missed. You also help Google do the same thing....
Its well worth the read The Importance Of Deep Linking