Rubber washer on Coleman Mach AC unit

[dsisemore1]

On my Coleman Mach A/C unit I have a rubber washer on the fan blade shaft next to the fan motor.  I have two a/c units and on one of the units the washer has deteriorated.  I am trying to determine what is the purpose of the rubber washer? I have enclosed pictures - one is of the good washer and the other shows the deteriorated washer.  Any ideas what the purpose of these washers are?

[DancinCampers]

On my units the fan shafts a vertical, and the seal prevents water from migrating into the indoor blower fan area.

[StirCrazy]

could serve a few purposes, I would expect it is to prevent water from migrating down the shaft and getting into the motor.