Here is the scenario: you have a metric is required to change DURING the time when the actual load test is running. You can't simply set it before the test starts like you would with a regular User Input field. An example could be user think time on a loop, which you need to change from 15 to 25 seconds DURING load test execution.

It is possible to do this thanks to the "Enable Value Changes" checkbox!

Here is the difference. It is possible to view a load test that is running by going to the Load Test tab -> "Jobs" page. Then click on the name of the running job (it's a hyperlink that takes you to the job). Then click the "Change Runtime Settings" Button:

If you have defined a User Input Field checked the "Enable Value Changes in Real-TIme" button above, you can change the value of that User Input Field:

But if you haven't checked that box, the "Users Defined Variables" section will not show up.

So, if you need to change the value of a user defined variable during load test execution, check the "Enable Value Changes..." box during the definition of the user defined variable. User defined variables that you can change during Load Test Execution have the Java System Property assignment:

 So if you see a user defined variable with the Java System property, you know you can change it during test execution!