Need help on :
1. How to integrate ServiceNow with Splunk?
2. What is necessity / benefits to integrate ServiceNow with Splunk?
There is an app (probably several) on SplunkBase.
yes , how it useful?
you can bring snow data to splunk and create tickets from splunk to snow
read more here: https://splunkbase.splunk.com/app/1770/
yes , how this app is useful?
May I suggest you give this a good read. It should answer most of your questions.