To improve the freshness of content
Freshness is one of the ways Google judges the quality of content. Therefore, regularly updating older content is a good way to get back on the search radar. Doing so can give you a ranking boost and subsequently increase traffic.
To Improve CTR
When the date that shows in the SERPs is from the current year, it makes a much better impression than if it is from several years ago. Regularly updating your content is a good way to increase your CTR.
Keeps Your SEO Up to Date
More frequent updates will get Google to index your blog more often. It shows that the search engine has to expect content changes more often so their crawlers will swing by more frequently.
Do More With Less
Older content that already has some traffic can more easily be put back on the map. Plus, re-using existing content takes less effort than creating something from scratch. It basically allows us to publish a new post without actually writing one.