> with that tiny display – to choose whether you run over hill A or around town B?

Previous devices I've had only did breadcrumb mapping, though you can match the trace you are making or the pre-programmed trace you are following with a paper map to make such decisions. I upgraded a while back to one that has actual map data so what you suggest is actually possible, though I use it to augment my paper map with accurate positioning rather than to replace it for navigation decision-making.

> … what you mean when you say you use the watch to plan your route?

“Plan” is probably overstating it. Most of the time I'm following a route and using it to make sure I don't go too far off course, or if I deliberately take a detour (“ooh, I bet the view from over there is pretty”…) to help me get back on course afterwards.

The watch does have enough map data and enough brains to plot a simple route to a waypoint, it can certainly reverse a trace to make a track “home”, but I don't use any of that. If I'm somewhere where I have a few relevant routes already in the watch I can switch mid-outing if I decide to do something longer/shorter, and if I go off my beaten track I can use it (possibly along with my paper map to make decisions about doing direct or around bits of landscape) to roughly guide me to a known place.

I do sometimes use the traces to plan future routes though, especially when I've been out exploring new routes or a completely fresh area. For example if I spun 180⁰ somewhere there is a good chance there was an obstacle at that point I might want to avoid in future, which may not be obvious on the paper map, but I remember when I see the past trace. Route traces can be nice to share too: I can hand send friend a route that I particularly enjoyed, and they can do the same back.

These days most trail runs hand out a GPX trace for you to more-or-less follow (allowing for number-of-points resolution issues on any given watch & such), but back when I started events would often just give a description. I would go out to do a recce of parts of the route following the description and paper map, maybe getting lost a few times due to missing a turn, and back home I could edit the trace to cut out those deviations so on event day I had a reasonably accurate path to follow (the watch vibrates if I go beyond a certain distance off-course, which I might do intentionally to avoid a temporary blockage like an angry looking bull in a field but don't want to do accidentally by, say, missing a somewhat hidden turning).

Some people use the location & speed tracking data in conjunction with heart rate, temperature, and such, to tweak their training plan, but I don't go into that detail.

> imagining a person […] thinking, "Oh yeah, that was a good run. Look at that part where I turned the corner onto Market Street! Hah, I remember that, good times."

I do have a plan at some point to make a big map of the moors & dales for my wall, with my favourite routes (from events or personal outings) marked on, the tracking data will be useful for that if it can be made to integrate nicely with whatever I end up using to draw the main map. Like the world maps people have on which they mark which countries/cities they've visited. I've had a fair number of good times out in the green. Less so for city outings or other road/pavement routes.