Why is floor leveling screed mix so dry

I've just put down the screed on my bathroom reno to give the floor some fall into the floor trap and was wondering why it needs to mixed so dry. What sort of a problem does extra water in the mix cause ? The only thing that I can think of is that less water possibly makes the screed more porous when it has set, so that if the water proofing is under the screed then any water penetrating the grout won't puddle just underneath the tiles. If this is the case then is it ok to lay the screed with...

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