Can anyone tell me what the purpose of the highlighted section is ?

It is in-between the carb and the air filter, on my other carbs it is just a normal spacer,
whereas this one has an inlet in it. Its a 1 1/4" carb with nothing
special about it other than that which I have nfi about.

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No more Minis!......For now.

I'd guess it connects to the rocker cover breather. Looks about the right diameter. Even more likely the case if it were turned to point upwards on the other side.

I've never seen one like that before, though.

Yep it is an adapter to connect crankcase vent pipe too

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4efte powered Clubman
supercharged 998 race car

Is their any benefit in using it ?

And what is its purpose in connecting to there ?

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No more Minis!......For now.

Its an emissions thing to try and stop crankcase gases from entering the atmoshere later carbs had a connector into the main body of the carb does the same thing

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4efte powered Clubman
supercharged 998 race car

You should have a hose from the rocker cover to the spacer. The rocker cover should be the type with the hump and inside the hump is an oil separator. On the rear of the motor the side cover (tappet cover, side plate, etc.) should have a pipe coming out of it and it should have a "squashed end," there is a very small slot in the squashed end. Your oil cap should be the sealed type.

How it works....
The carby sucks fumes from the motor via the rocker cover, the separator stops oil from getting sucked into the carby, a very small amount of fresh air is allowed into the motor via the slot in the side cover.

It is not a very good system! If your air filter is blocked it sucks harder from the rocker cover and sucks oil from the motor.

I have a brand new Mini K carb here, it has one of them suckers on it.
Early days emission equipment.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.