Freshening up old postings with new information adds value.
Oh, how information fatigues fast!
The "submit" button has hardly cooled when the blog posting you've just made is out of date.
Revisiting old blog postings and updating them with your latest perspective, and with new information, has a two-fold positive effect: (a) the value of the information is potentially increased and (2) search engines see the change as another element of your "rapidly changing content" requirement for Search Engine Optimization.