Why do some airports put 2 jetbridges on a plane? Is it necessary?
It is not strictly necessary. But it is done in order to save time.
More points of passenger loading leads to faster loading, potentially reducing delays, airport charges, etc.
In some cases, there is a separate jet bridge for first/business class.
For large aircraft like A380 etc, is quite common to use multiple jet bridges, as use of a single one may lead to quite long loading time.
The reason they use 2 or sometimes even 3 (A380) for wide-body aircraft (I have seen 2 on a B757, but that is rare) is because the more entrances to facilitate faster boarding and unloading passengers from the airplane. It saves much more time than loading the plane from one jetbridge and in the case of the A380, there are 2 full wide-body decks to fill. Passengers also like getting on and getting off the plane quicker as well.
Interesting that there is this similar question on travel.se
So that economy class passengers can also see at least one of the higher classes (when they exit) and wish they booked that! :-/ Trust me you sure will wish that and that someday can mean more
$ for the airline
Joking aside I have seen it happen on many airlines. The people on left Aisle go out from the first one and people on the right aisle take the longer route to exit from the gate that's closer to cockpit.
Primary reason must be to transfer people faster to make the jet available for crew as soon as possible specially after a landing when the jet has to fly again in an hour.
In addition to Aeroalias' answer, some airports adopt a Multi Aircraft(Apron) Ramp System.
The gate may be used by multiple small or a single large aircraft. It allows airport planners to make their gates more flexible and efficient. In such a case two or more jet bridges are needed for a single gate.
Some of the busiest airports including Beijing Capital, London Heathrow and New York JFK have already adopted such a system.
More details can see this article.