Last visit allows you to target a client based on either specifically a check-in, OR interaction (check-in or purchase)!
This is a very commonly used condition, and is used in several contexts:
Within autocampaigns, it allows you to capture a client when they go a specific amount of time without visiting. An example entry rule would be, last visit is exactly 30 days ago, to capture someone when they hit the milestone of not visiting within a month.
In segments, this allows you to create a one off list of clients who haven't been in a certain amount of time to retarget them. A great example is if you're running a sale and want to target clients who haven't been within the last 6 months, you can use last visit is at least 180 days ago!