1) Staines Road is a primary road (A315), not a residential road. 

2) Because someone chose to map them. As with the individual orchard trees you deleted a couple of years ago, a feature not being mapped the way you would do it is not an adequate reason to delete it. The feature not existing or being otherwise factually incorrect would be, but that is not the case here.

I would not usually map separate sidewalks on a residential road, unless it was a new development with a lot of crossings with accesibility features. However, another mapper did and the features exist.

On main roads with defined crossing points, separate sidewalks are useful for pedestrian/wheelchair/VI routing, particularly where accesibility features and obstacles can be mapped in more detail than crossings as a node on the highway only.