One Thing More
I didn't mention this 19th Century brewery gossip because I wanted to be sure my answer remained under 300 words, but supposedly the reason Maerzen beers were served at early Oktoberfests was because the brewers were getting rid of their left-overs.
By selling and serving the last of the Maerzen beer, they emptied the storage barrels and freed them to be filled with newly brewed beer at the time of year when the weather usually allowed them to resume brewing.