Firstly, these Mock Tests help in ensuring that you finish your actual JEE Main exam within the stipulated 3 hours duration. It creates the pressure condition in your mind to complete the test within the stipulated time.
Secondly, NTA conducts JEE Main exam in computer-based test (CBT) mode and not in pen-paper mode. You need to build your test-taking skills in CBT mode with these Online Mock Tests for JEE Main to ensure that you are comfortable in taking the test in CBT mode.