I now see the bypass you are talking about and read about all the reasons these were installed.
My system is 100 years old but the boiler is a Burnham V3 and the burner is a Blue Angel, both from the 1980's and professionally installed at that time.
According to my original Burnham V3 instruction sheet, they simply show a supply and return pipe as the recommended installation for a gravity system with no bypass valve. However they do mention the need for a bypass in the situation where a gravity feed system is "converted". Mine is not converted... it's all original (no pumps).
My guess is that since water is flowing so slowly and has such a huge mass that there is little chance for water temperature differential to get so great as to shock the boiler. i.e. water returning from my radiators is never cold unless water flowing into them from the boiler is cold.