What are the use of Critical Updates in Salesforce?

Salesforce periodically releases updates that improve the performance, logic, and usability of Salesforce, but may affect your existing customizations. When these updates become available, Salesforce lists them in Setup at Critical Updates and displays a message when administrators go to Setup.
Step 1 : Go to Setup
Step 2 : Enter critical updates in quick find box
Step 3 : Select Critical updates