I am a customer, I run a bunch of personal sites and hobby projects. updown.io is wonderfully cheap, the pay as you go model is perfect for my monitoring needs. I have no need for the stats, just the monitoring which it does well.

Just waging a guess here, but people / businesses who depend on data from those websites?

I'm curious too. Tracking your own website makes sense but there're definitely better ways if that's your own website. Why do people want to track other website regularly?

I think you're giving too much weight to what's in their example image. I use a competitor's free tier to monitor my own stuff. Using a SaaS is better for several reasons:

  1. Don't have to install / maintain anything.
  2. Who monitors the monitoring?
  3. Monitoring from inside my network doesn't always fully approximate end-user availability.

That's pretty much it, my customers are people who have websites and want to be the first to know when there's an issue on it so they can fix it, in which case a SaaS solution is usually better than some internal tools because it eliminates setup/maintenance, keeps working when all your infrastructure is down and monitors everything (including internet link). Some other clients monitor website they do not own when they depend on it, for example as a vendor I could monitor Amazon if I sell products on it to be aware of any issue, or if I'm a digital customer engagement platform (what my full-time job does) I can monitor services I interact with like facebook API, twitter API, etc.