What are lightning events used for?

The answer to your - what are lightning events used for is -

• In simple terms, Events are used to establish communication between components. If you want to send the data from one component to another component, we use events.
  
• You fire an event in the source component and this event will be handled by any other component depending on the type.
  
• Let's take an example of Account Search. In this scenario, you need to have an input field to enter the search string and you need to display the result in a table.
  
• Component 1: create an input field and a button to perform a search. Call the apex and get the data. Component 2: Display the result in a table.
  
• You will get the list of records after clicking on a search button and you need to pass these records to component 2 to display. Here we use Events.
  

2 types events: Application Events and Component Events

• Application Events are used to establish communication between two or more independent components.
  
• Component Events are used to establish communication between the components which are in the containment hierarchy.
  
• Note: Ofcourse, you can do this use case in a single component. I just took this as an example to explain the events.