As long as you shit in your home, you need it.

Actually 75mm is not just a vent pipe. It is a sewage pipe. It is straight connect to your main hole and many branch out to toilet and sink.

Nope. It is actual the sewage pipe. Very easily see. Check your vertical pipe in the toilet where it go up. Than go outside and see the roof vent pipe location.

Soil and waste pipe is from the kitchen (sink and floor trap). That is direct to Main hole and no vent. It is correct.

Bathroom sink, floor trap and wc is to sewage pipe 75mm. The vertical sewage pipe go down is to main hole and up to roof is to vent.

This is from what I explore in my home. Include checking inside the main hole.

Sure lah. New house don't have anything 1. And it fascinated me to see inside.

But warn you. Don't simply open it, if many many yrs. Bad experience for me.

I once open a man hole at my working place. Fark, hundred of hundred of bugs craw out from the man hole and run everywhere. Just like you see those movie.

Sheltox and burner finish up also can't kill them all.