However, it does provide vast tracts of land without a building on them. Of course none of this is unaltered habitat, but many farms have hedgerows which do make for interesting habitat. And certainly great for walkers and birders. They are often signed, at intersections with roads, and are marked on ordnance survey maps. They mostly go through private land, but because they are so old, everyone accepts them.
Landowners allow the public to walk through their land.
Of course the walkers are meant to stay on the trail, not damage anything, and close gates. In many cases gates are unnecessary because of all the stiles. We walked through sheep fields in Cumbria using an ordnance survey map. Sometimes, when going through a field, the path was not obvious. However, when we got to a stone wall, we always knew we were on the path when we saw a stile built into the stone wall.