Garmin Speed sensor

I have a Garmin 910xt that I use for the bike of course GPS keeps track of my speed. I decided to get the rear speed sensor that goes around the rear hub and should come today. So I realize that indoors it makes sense but what about using it outdoors with the GPS. My understanding is the speed sensor will overrule the GPS speed data. My years of using this it is pretty much dead accurate for distance so speed should be deadly accurate. I have done many long rides and hit the same mileage marks almost within 10-50 feet for 20-40 miles.

I also bought the cadence sensor and have never used a cadence sensor figuring I can count my cadence anytime I want. So do any of the group use a speed sensor when riding outdoors? I guess you can hook it up with Zwift but I have never used Zwift don't see the benefit as long as I know my speed on and heart rate on the trainer. I have a cateye at the moment set up for the back wheel on my trainer bike. I give this garmin a go. Some how Zwift measures power but I cannot figure out how that could be anything more than an educated guess.
Deacon Mark............in the very cold midwest