When building some infrastructure, in this case a railway, that is close to a mountain slope, it is necessary to cover the mountain slope with a wire mesh.
The biggest reason for that is the safety.
The steep mountain slopes can be very dangerous, because the rocks undergo the natural processes of weathering and erosion, which causes the rocks to break up and fall downwards. The railway can be locked in that way because there will be rocks on it, so the trains will not be able to pass further. But what is more concerning is that rocks may start falling directly on the train itself, or on the railway but when the train is passing through, and can cause a tragedy. The train may get pushed of the railway, or the rocks may knock it down or get through the windows, so lot of lives can be lost, as well as lot of damage will be done too.