Puzzled about function of DPM under concrete slab

[mikeforward]

Hi everyone. I'm a new member looking for advice.

I've just put down a concrete slab for a wooden garden room. I watched a good number of Youtube videos and then did the following:
- Dug out to 17.5cm then put down 10cm of MOT type 1, then compacted it thoroughly
- Put down about 2.5cm of building sand and levelled it
- Built formwork 15cm depth, all perfectly level
- Put down 1200 gauge DPM extending up and over the top of the formwork by about 50cm on each side (being careful to ensure it wasn't punctured at all)
- Put down steel mesh supported by meshmen
- Poured the slab to 15cm depth, tamped it down etc
- Waited for it to dry

It's now been 5 days since the pour, and I'm very happy with the result.

Here's my question: I'm puzzled about the DPM. I understand that it is there to prevent moisture coming up from the ground into the slab and then upwards into the wooden structure. But here's what I don't understand:
1. No-one wants to see ugly black plastic sticking out from the side of their garden room slab. So at some point you surely have to cut off the DPM at or below ground level (so that it can't be seen). But once you do that, won't it allow moisture to penetrate the slab from the sides and then travel up into the structure etc?
2. If the slab is bigger than the garden room (which surely it should be: why would you want to put the main load right on the edge of the slab, its weakest point?), then when it rains the exposed parts of the slab will absorb moisture anyway, and some rain will inevitably pool on the slab and run down the sides between the slab and the DPM, and then be held in place by the DPM, which could therefore even be making things worse!

Any wise advice gratefully received

Thanks a lot

Mike