Adding links to your already published posts have two advantages:

1. Adding links to your post related to the topic will help you in On-page SEO. On-page SEO is like adding links to your posts, using the right keywords, the repetition of keywords in your article.
2. Adding links to your post will convert a single reader to multiple views. The reader might be interested to know more. Let say, you are writing about CoronaVirus symptoms and you add a link to your post about the importance of the N95 mask then he/she might click on that link to.

I hope this will helps you.