Useful to know if you are the only one who has a service outage from Telkom, Eskom, Twitter, your bank, your ISP, your mobile service provider, etc. You can check as well as report on the site but note it does not replace official reporting to the service provider. Some service providers do monitor and respond on the site though.
"A single person reporting a problem doesn’t constitute a large-scale outage. To make sure that outages are correctly represented, we calculate a baseline of typical problem reports for each company that we monitor. By verifying that issues are affecting a large group of individuals, Downdetector only reports an outage when the number of reports is significantly higher than the baseline."
They have mobile apps for Android and iOS too.
See Status overview