What's the purpose of this blue plastic?

[Wonky&old]

It's behind the render, which is behind the cladding and did stretch all the way under the patio, until I cut it to address the grade. The first row of slabs were sloping towards the house. One end of the blue stuff was letting water in where it had been cut previously. Any advice on how to deal with this would be gratefully accepted. I'm thinking to dig down and fill with gravel to give the water somewhere to go. It was pooling up against the house.would.i need to replace this blue stuff or can I just let water drain into the proposed gravel channel. House is around 1920's with a remodel roughly 20 years ago. Thanks!

 

[Cubist]

It's a Damp Proof Membrane and in this case its purpose is to prevent moisture migration into the fabric of the building beneath the render. You need to make sure that all rainwater run off from the wall is directed away from the house and not allowed to puddle in front of the wall. The builders should have installed the slabs to slope away from, not toward, the wall unless they were directing that water to a drain.