This is because there's always room for improvement. And who doesn't want to have the best possible blog content for readers to enjoy?
Also, just the fact that your blog has been spruced up somewhat will help with SEO. As all bloggers know, Google rewards you for constantly adding completely fresh material. Rewriting of old blog posts is not unlike this. It's another way of making Google know your blog has a pulse, even if you haven't created entirely new content for it.
Things you can do
And there is always something you can do to improve an old post. For example, you can tweak its keyword targeting by putting it through a word cloud generator like Wordle. And you can apply similar changes such as including subheadings if they weren't there before and linking out to quality related sites if you hadn't done so originally. All of these things are known to lift a post's position in the SERPs.
Longer posts are better
Adding more thoughts to make some old posts longer than they were before is a good idea, too. Of course, you don't want to make this fluff. It should always be good, useful content. You can also correct some points you made that are no longer relevant. That's worth doing for any blog niche, but it's particularly useful if you write about fast-changing disciplines.
If you add some good new observations you can cut bits here and there and the blog post will still be the same length as before -- or a bit longer. And word count does seem to be an important ranking factor, with longer blog posts performing better than short ones.
Rewriting a blog post in such a way will make it more densely packed with good information. Needless to say, readers will appreciate this and be more likely to return. There's a greater chance they'll share such content on social media, too.