What is the use of test.isrunningtest() in Salesforce?

The Test. isRunningTest() method is used to identify, if the piece of code being executed is invoked from a Test class execution or from other artefacts such as a Trigger, Batch Job etc. Returns true if the code being executed is invoked from a test class otherwise returns a false.

Returns true if the currently executing code was called by code contained in a test method, false otherwise. Use this method if you need to run different code depending on whether it was being called from a test.
Other scenarios:1) To ensure the trigger doesn't execute the batch if Test.IsRunningTest() is true, and then test the batch class with it's own test method.2) Testing callouts - in your callout code you check to see if you're executing within a unit test context by checking Test.isRunningTest() and instead of getting your callout response from an HttpResponse.send() request, you return a pre-built test string instead.

The Test. isRunningTest() method is used to identify, if the piece of code being executed is invoked from a Test class execution or from other artefacts such as a Trigger, Batch Job etc. Returns true if the code being executed is invoked from a test class otherwise returns a false.