Immediate use case--there's a piece of trash beside the trail not too far in from the trailhead. I want to pick it up on the way back, I do not want to lug it multiple miles into the backcountry. Unfortunately, while it's clearly visible to an outbound hiker I have yet to find it on the way back.
Secondary use case--marking non-obvious trail features. Stuff like GaiaGPS will let me drop waypoints, but I'm after something that will notify me as I'm approaching.
Yes, this will obviously keep the GPS powered up--but it's going to be powered up anyway. Fully local operation is mandatory as service out there is patchy at best.