I don't really plan my walks, except to try to find out where the actual point of a route is, or how to navigate my way through a small town or village where the signposting is poor. Streetmap can be useful in these situations. I hate spending time wandering around looking for the way forward. Other than that I just check the route, note some of the towns and villages in the direction I'm going and then go. I camp so really don't have to bother with too much detail.
I often use Streetmap when I want to look back and 're-live' a route I've already done. I can bore my wife silly with my "I've been there, I sat on that bench, I had a pint in that pub, that's where I had a prawn sandwich, look at that view!" and so on.