A blog gives you an online platform to share your story about your products and the process you go through to create such unique pieces of work to build your brand. Blogging is incredibly popular and it is a powerful communication tool.
Blogging can help you market your iPhone/iPad app in several ways. There are many blogs that do iphone/ipad reviews where you can submit your apps. These sites are already established and if they can mention / review your apps, it will be helpful for your apps. Do send promo codes to them. Search the blog sites and post the details of your app.
You can also create your own blog sites to discuss and showcase your apps. You can also post videos, tutorials and graphics to your blogs and promote your apps on your blog. You can also integrate blog in to other social media such as face book and linked in. So more and more people will visit your blogs.
Interesting blogs can convert bloggers to paying customers.