Should I add Structured Data on a Website that won't show them on SERP?

Rich results do not appear for the keywords I want to rank for. Should I still add it?

Knew someone who had rich results not displaying, but after implementing structured data, the overall visibility of the site improved. Remember, SEO is not just about immediate visible returns. It is often the "behind the scenes" changes that are crucial. So, yes, I would still add this.