“You can take an ECG at any time, when you’re feeling symptoms such as a rapid or skipped heartbeat, when you have other general concerns about your heart health, or when you receive an irregular rhythm notification.”
You don't have to check it at all. It checks automatically without your having to do anything. But that won't stop a lot of people who now have something new to be concerned with.
As have I. Checked it, saw the neat little graph, said "Oh, how cool!", and moved on. Another neat little feature to be used at parties, and other informal gatherings to show off my watch; other than that... Meh.