Internal links in blog posts are important because they help Google find your blog posts. If there are posts in your blog that have not been crawled, then Google and Bing can crawl them from other posts where they are linked.
Internal links also help drive traffic from one post to another which increases conversion and user time spent on your blog. If Search engines notice that people are spending more time on your website, then they will consider your post relevant which leads to search rankings increase.