GMAT Club Tests - Free Access on Holidays

I would suggest that you do not take it if you have to ask that question. I would recommend that you do half the work and hope for the best on the test day
I assume you are playing devil's advocate.... The point is to strain you beyond what you may encounter on the test day. Similar to how may be done in training/competition/etc - you want to make it hard on you in training/preparation so that you would have

The idea is to prepare you to the unexpected, which happens quite a bit past the 700/Q50 level - you encounter questions never seen before that you are not prepared for.
So the questions that you will encounter in the GMAT Club test will be on average harder (none of them are out of the scope), so on average you will get 30 700+ level questions and 7 600-700 level questions whereas in the GMAT Prep you will see a few questions that are 500-level, and 10 or so that are 600-700 level, and only about 20-25 questions that are 700+ level.

Also, on the GMAT Prep if you are not getting Q50, you will be getting quite a few more of the 600-700 level questions. GMAT Club tests mostly have 700-level questions, so even if you are scoring Q45, you will still get "bombarded" with more difficulty questions. That's why I would not recommend these tests to someone who is not at Q40 or even 45 level.