With a speed sensor the auto pause / auto resume is triggered faster. And your computer will display 0.0 when stopped. I only have the speed sensor on two of my bikes and never miss it on the two that don't have it. The only reason I even have those sensors was from when I had a non-GPS ANT+ computer.

I move the magnet out of the way when using my road bike on rollers since the speed is so high its silly to count that distance for anything and all I'm interested in on those rides is HR, cadence and time.