CG, where do you want to start?
I have an SD card in the back, purely because I have some maps on it. Not OS, because they cannot be put onto it, and not GARMIN TOPO because they are cr@p, but some free maps from tinterweb. Can supply a link if you want. They are nearly as good as OS, and quite good enough for my use. The unit itself without the card will hold all the data I ever need to store, prior to down(or is that up)loading to my PC. Others use bikely, bikehike etc, but I've used OS maps all my – very long – life, and feel comfortable with what the OS produces.
I plot routes on Memory Map – other mapping software systems exist – and transfer them to the unit. I follow the directions from the unit when out, and although I always carry a paper map as back up I seldom resort to it. I record my trail and down(or is that up)load it back to my PC when I return, so that I have a track of where I've been. Very useful for finding out where I went wrong, or where to alter the route for next time. Others use the "trail" facility, but I don't find these as easy to follow as the big compass arrow, which just points to the front all the time and is an almost instant check on being on the correct route.
I can make a "mark" on GPS of interesting points and show these on MM on return. Can be useful at times.
A Legend will do so much more than I want it to, but that's true of any electronic gizmo. Really I use mine as a direction pointing tool, with the recording facility for when I get back. I'd be lost without it. Care is needed plotting routes before my rides, but it saves having to stop every five minutes to get the map out.
IIRC there's not a great deal of difference between the Legend and the Legend H models, apart from the vastly superior reception, due to a totally different aerial being fitted.
Enough? If not mail me.