My understanding is that the thin syrup has to be spread out to dry and stimulates cleaning  of cells that then are ready for brood. 

Thin syrup also more resembles nectar in concentration than stores and suggests a flow.  

Additionally,  increased humidity in a dry environment is known to stimulate brood rearing. (Pirker)

With thin syrup, there is no need to gather water to feed  brood.

The problem is how quickly thin syrup can spoil, this the traditional method of feeding overhead  in one-pound honey pails with a few nail holes in the lid.

Apparently a little goes a long way.

Pete can probably find some 100 year old articles about this forgotten technique.

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