It is just a plug taking up the space the DuoTrap speed/cadence sensor. Depending on a lot of other things, some of which include what GPS/cyclometer device you have, what your future plans are for other things you want to get such as a power meter, you might not want the Bontrager DuoTrap.

A crank or pedal based PM will also give cadence. If a PM is in your future desires. And you might find for distance, your GPS is more than accurate enough for your needs and not need a speed sensor. Or prefer to get just a wheel sensor which can be less than half of that price for the DuoTrap.