For people who have been to Goa before, this would be a familiar mode of transport and probably developed because of the need to beat the traffic.

It can be recognized easily as the Taxi Bike owners wear an Orange colored jacket.
It is quite a popular mode of transport amongst the locals and serves a very purpose of connecting small distances. However it is suggested not to be used very frequently as it may not be safe sometimes because of the reckless way the bikes can be driven.
Rates can vary and have to be negotiated first. Bottom line is that avoid using them unless you have to urgently get past the traffic and your destination is nearby.