First, realize that the energy coming off the rear of a woofer cone is exactly the same as the energy coming off the front of the cone, but 180-degrees out of phase. The cabinet is sized and the port tuned to extend the bass response of the system. Some of the energy coming off the rear of the woofer is turned into heat through the use of acoustical foam or stuffing. The remainder flows in and out of the port as the woofer cone moves in and out creating higher and lower air pressures within the cabinet itself.
So you can imagine there is a fair amount of energy moving in and out of the port. If the area of the port is too small, "port chuffing" results.
The "best practice" is to have the port located on the rear of the cabinet so that any port noise is directed at the rear wall and not the listening position. But this means that a fair amount of "breathing room" is required to prevent bass from piling up at the rear wall and causing the sound to become "boomy." Thus, the speaker needs to be located a reasonable distance from the rear wall. Normally this would be about 18 - 24" minimum, the more the better.
Often in a given room, this is not possible or desirable. In this case, front porting is often used so that the speaker can be located closer to the rear wall. In these cases, the area of the port needs to be increased such that you minimize any potential port chuffing. Since the area is increased, the port also needs to be elongated in order to maintain the same cabinet tuning.
In the end, front porting means the speaker will be more flexible in terms of placement in the room and represents a flexible alternative to rear porting.
I hope this helps.