Used to check jmeter properties ,system properties and any variables that we defined at test case level or any other variable passed by any request to the server or any variable passed as a response back from the server etc.
Debug sampler can also be used if we add a csv data set config to read values from excel file and to see if values are read correctly
In test plan click add and add a user defined variable and give a name as url and value as google.com
Then we add a Debug Sampler and run the test
We run the test and go to results tree and click the debug sampler and in response data and response body we see name as url and value as google.com
So to conclude debug sampler can pick all values whether they are defined in test plan variables or coming from csv file etcso it is good for debugging to check if we are getting all the values correctly or not