Tagging other people or places in your Instagram posts can be useful for a number of reasons:

1. It helps build relationships by supporting the Instagram profile of the person you’re tagging.
2. It notifies the person/account you’re tagging, and therefore can keep you top of mind for them.
3. It shares accounts / products that you like with your audience, and allows them to find out more if they’re interested.
4. It demonstrates that your account is not “all about me” but that you’re interacting with other people.

This video (not quite 4 minutes) walks you through how to tag people and places.