[QUOTE=writerguyfl;23012779]Just speculation, but I would guess that a formal tip envelope is there to eliminate the "is that a tip or just money left on the table?" that Housekeepers sometime face. QUOTE]

This. In hotels I have stayed at with the envelope, if I left a tip on the bed or anywhere but in the envelope itself the few dollars will inevitably still be sitting there when I get back to the room.

If I had to guess, there was probably a precipitating event at any hotel that uses these tip envelopes- i.e. some guest claiming that housekeeping "stole" the $2 that the guest just happen to leave sitting out for no apparent reason