Apache Storm is used for real-time analytics of a large volume of data. Apache storm is used by companies like Twitter, Spotify, Wego, Yahoo, etc. Twitter uses a storm for real-time analytics, search optimization, etc. Yahoo uses Apache storm for using their big data with fast processing. Wego is a travel-related company that uses Apache storm to reduce concurrency issues for a better experience to users. NaviSite uses the storm to monitor event logs. The storm will go through all the messages and if the message is matched then only it will be added to the database.
If you are interested to learn Apache storm, I recommend this Apache Storm training by Intellipaat.
You can watch this video on Apache storm tutorial to know why is Apache Storm used: