What's the point of this screw (rear mech)?

Appears to move the cage forward/back (does it?) Why? When?
TIA

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It’s moves the cage up or down to give more engagement to the cassette .

It's called the b-screw, it moves the upper jockey wheel closer or further from the cassette. See here for more info.

Ahhh - so it's 'engagement' with the cassette that's the function? May I ask why one may want/need more/less? Whats the optimum/standard?
Thanks very much

its to adjust the space between the upper jockey wheel and the cassette, if it is too small then the chain rattles between them, if it is too great the shifting is not so precise

In reality not very much, meant to bring the top jockey wheel closer to the cassette sprocket so that when shifting the chain is at a more obtuse angle therefore encouraging the chain to move across to the next sprocket.

Many thanks - I can see a use for it perhaps when changing cassette/chain/speeds, but all the time when setting up in the stand to slicken up changing I guess.
Cheers 👍

While most lowest sprocket is 11 it can also be 15 and you would want the top jockey wheel close hence backing of the screw

Ideal - I'll have a little adjust and move the jockey wheel up a touch on all my bikes today👍