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So I have been wondering about this for a while. Most keys (I'm thinking Schlage, Sargent, Yale, etc) have a top and bottom shoulder. These are all shoulder stop keys, so I fully understand the function of the top shoulder. However, I cannot think for the life of me of a good reason for a bottom shoulder. Now, in about 1983, Corbin did away with its bottom shoulder because it interfered with the face of the cylinder when X class cylinders changed from .509 plugs to .552 plugs and a recess was milled in the face of those plugs. I sometimes buy NOS Sargent blanks on eBay and the occasional one will have no bottom shoulder. Also, some old push key padlocks had keys with no bottom shoulder so it could be pushed in further in the lock to open.

The only "good" reason I can think they used bottom shoulders is because it visually balances the bow of the key, some of which were very ornate in the past. While I do find old Corbin X class keys with a cloverleaf bow and a bottom shoulder more visually appealing than one without the bottom shoulder, I do not find they operate differently...

Most keys here only have top shoulder.

Often keys that have both shoulders fit into locks that have a cutout for the shoulders and its the shoulders that turn the plug taking any stress out of the warded part of the key, a better system resulting in less broken keys.

The material in that little bottom shoulder x10000000 adds up to a lot of money to a key/lock manufacturer.

I think its just a throwback to old designs not updated.

Relying on bottom shoulder has bitten me in the butt several times. Top shoulder always if possible.

I have done the same by error and have been bitten accordingly... I'm leaning towards the fact that the bottom shoulder was there for aesthetic reasons only (symmetry) as I cannot think of a practical reason for its presence.