Different strokes for different folks.A,b, and c nice but something I refuse to worry about when I am in the woods.
The elevation (altitude) derived by the GPS multiple triangulation is just as accurate as a barometric altimeter which can be affected by changes in the weather. I've used several barometric altimeters, and one thing a person learns is to check it before bedding down, and reset it in the morning, if the barometer has risen, or fallen during the night..... The one you didn’t list which is about my only fancy gadget on my watch is an altimeter. If I was still in Florida an altimeter would be pretty pointless but in places with real mountains they are nice to check against the topo.
The two main pieces of information a GPS gives you are your geographic coordinates, AND the elevation at your location. I didn't mention the altimeter function, or the coordinates function because that's just basic info.