The problem you can encounter in well trodden areas, such as the Lakes and Snowdonia, is that there is a plethora of paths, some are mapped, some aren't, and some are mapped wrongly. The main advantage of a GPS is that it facilitates the exploitation of these minor tracks as well as off piste walking, especially in reduced visibility, allowing you to explore the areas the crowds avoid. I also used my GPS in the Alps this year where I found it an invaluable nav aid on ridge routes to ensure the selection of the right path and to monitor progress along an unfamiliar ridge.
Yes you can do it with a map and compass etc, but a GPS is just so much quicker and easier, and does not actually involve blundering along looking at a screen. A quick glance from time to time is all you need to stay abreast of progress, position and route ahead whereas a map often brings you to a dead stop while you refer to it.