Many roads have a white line or ‘edge line’ painted along the outside to help identify the edge of the road, nothing more. It’s often painted where there is no kerb to distinguish the road surface from the verge and where there’s a greater risk you might run off the road. It’s also used along roads prone to fog and mist, heavily trafficked roads where headlamp dazzle is severe and at sudden changes of carriageway width.
The edge line is particularly useful on bendy rural roads as it helps draw you towards the edge of the road, rather than the centre line and it improves your assessment of the curve, reducing steering fluctuation around the bend. Having a road marked out so clearly also helps reduce your mental workload. It’s not illegal to park on an edge line and you may have noticed on some roads, particularly in 30mph zones, that the edge line is painted quite far away from the verge or kerb. This gives the impression that the road is narrower than it is, which has a natural speed calming effect.