Suppose you liked the company, but the timing or the offered position wasn’t good for you. In that case, you can offer to stay in touch, add them on Linkedin and keep an eye on possible new opportunities. If you don't think it's necessary, that's okay too! But it can be an excellent way to grow your professional network.
Take a look at the video below (go to 4:00) to see Lauren Berger, the Founder of Career Queen, explain why it’s a good idea to stay in touch with the company.