As a sibling commenter said, the Instinct 3 Solar only does breadcrumb navigation, it doesn't do topographic maps on the watch (there are some Connect IQ apps that can add mapping, but you don't get good integration with workouts).

I use them all the time when cycling. I often plan a route, but when some different direction looks more interesting, I can spot check whether it leads to bike paths that will eventually merge back into my grand plan, erm, route. Or sometimes even for following the route, you want to look ahead by quickly zooming out or get a lot of detail at some complex intersection, where having a full map gives you much better orientation.

Well, except on a Garmin, my Fenix 8 is often so slow that I had to pause cycling to zoom in/out (even more complicated by not being able to do gradual zooming because it does not have a crown).

Yes, I know I can also use a bike GPS or a more generic GPSr with a large screen. I have used their gpsmap line since 2010 or so and even have the gpsmap H1. But having to always carry it around when you have a break somewhere is a drag and I always have a watch on me anyway. So I primarily use the gpsmap for geocaching and switched to using a watch for other activities.

but because they have something sipping tiny amounts of power so that I can get a battery life of several weeks

Coros watches have several weeks of battery life and fast maps. It is laziness (or margin maximization), because they could reach the same power budget by moving to a processor that is on a smaller node.