The only reasons I know are to protect the seeds and seedlings from insects, slugs, birds, drying wind and excessively fluctuating temperatures and humidity.
Plus giving them cleaner soil with fewer pathogens.
Plus maybe to get them to start a few weeks earlier, due to the jug keeping them warmer than the soil would.
I think of "spring-sowing" annuals as a variation on starting them indoors in trays. Not as warm, but maybe easier.
Actually, I think that both winter-sowing and 'spring-sowing' are actually forms of a very old-fashioned practice, sowing in an outdoor seed bed enclosed in a cold frame ... just the WS "cold frames" each have a volume of one gallon, instead of several cubic feet. And they are plastic instead of wood and glass. And jugs have a lower overhead and shallower root zone.
