Membrane Question

Our neighbours are having a area of traditional cobbles laid. The old base has been dug down (not to the bottom - it must be quite deep) and a new one constructed using a couple of inches of the old material (is this good practice?) then a membrane and then a couple more inches. All this is compacted before concrete is laid (a few inches) and finally the cobbles.
What is the purpose of the membrane? Under all that concrete it can't be to stop seeds? The only thing I can think of is to stop roots penetrating through all the old base into the concrete layer. Can anyone help?
Thanks
Harry
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The membrane and loose rubble is laid to give water a low resistance soak away rather than gathering and washing the soil out from under the foundations. It does do other things like stop root and weed growth, but the main thing is for good drainage as far as I know.
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I would understand that if it were block paving set on sand, but with concrete and mortar ....

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