Zwift awards you a boost when you cross a line (start/finish, KOM, sprint). Boosts can be weight reduction (15 seconds long, worth a few kg), aero (30 seconds, worth a second or two over a 30 second super hard effort), drafting (30 seconds, like drafting a truck if you're behind someone, otherwise it's nothing), etc.
If you don't use the boost (press the Space bar to use it) then at the next banner you don't get another one.
If I get an aero boost I feel obligated to save it until the Green Jersey sprint segment. Of course when I soft pedal through stuff I don't get the aero boost, and when I do a "okay this is the last sprint of the night" sprint I inevitably earn another aero boost. I've often ridden another 30-35 minute lap to use up that aero boost and I've extended a ride an hour at least once when on that "really the last sprint of the night sprint" I get yet another aero boost.
To give you an idea of what it's like during one of those sprints here's a short clip I made of a Green Jersey sprint on Watopia, the Zwift island. I got an aero boost at the prior banner (the KOM) so I was psyched to have it for the one sprint I planned on recording.
Screen shot from the ride.