2.3 litre exhaust flapper

[88 Capri (2022 SOTY)]

I was doing some reading and noticed that I should have one of these. I brought the exhaust pipe to the bench and noticed all that is there is the shaft.
My questions are what purpose does it serve, do I need it and how would the engine run without one.
I'm curious on my last question because I had trouble getting the engine to idle below 900 rpm and was thinking it might be something with backpressure. This is my first inboard boat so please excuse my noobness
Thanks,
Jim

 

[Scott Danforth]

it is there to prevent hydro-locking should your motor ever run-on (common problem) or you back up quickly

 

[88 Capri (2022 SOTY)]

Oh ok makes sense thank you.