The Importance of High Availability

Stock trading app, Robinhood was down for most of the day. Many of their traders were furious.
And many more applications such as energy, utility and traffic control etc. that directly affect our daily lives cannot be down and should not be down and when they are down due to force majeure, they need to back up very very quickly.

So, high availability is extremely important for certain applications.
Any insight, thought to share on this subject?