The original purpose, back during the time of the first AC game, was so that the hidden blade could be used more easily without obstruction or risk of accidental injury. The blade would extend extremely close to the ring finger. Since the hidden blade design was modified at one point, what happened in the movie may have been ritual. Or, perhaps the modifications are not part of the movie's canon.
shareWeren't the first modifications made by Leonardo DaVinci, so about a hundred years after the events of the film? But still Aguilar had blades on BOTH banda, but only need to sever a ginger from one...
shareso about a hundred years after the events of the film?
Originally the Assassins wore one hidden blade on the left wrist. The blade would go through where the ring finger should be, so the Assassin needed to sever the ring finger to use the blade.
The blade was later modified to not go through the fingers, making it more practical, easier to conceal, and able to be worn on both hands. By this point, the severed finger had become a sign of loyalty to the Brotherhood and removing it lingered on as a ritual.
Over time, this ritual was modified as the Assassins' enemies could use this to identify them. For example, the Italian Renaissance assassins used a brand on the finger instead of severing it completely, though evidently the Spanish Assassins were still using the old ritual. Eventually, even the brand was abandoned as even this was being used to identify Assassins.
"So it goes" -Slaughterhouse Fiveshare