It's significantly better than a Y-pipe mingling the gasses together and then randomly & abruptly spliting them apart again!
As for why...
The higher velocity would promote better scavaging as the exhaust gasses are getting away from the cylinders faster. This not only allows for less back pressure directly after the cylinders (better engine efficiency), but it has as stronger effect on pulling the following pulses along. (better scavanging)
Then take out the 4 90 degree bends around the tank and you are even further ahead on both counts.
