Phoenician traders show a similar configuration.
In their case, the reason was simply that ships were dual-use (trading and war); when at war, usually as part of a war-fleet, they mounted bird heads on the stem and stern-posts. When trading, they left the heads off.
Vikings appear to have had a similar arrangement: when going
viking they fixed dragon heads to the posts. When trading, they left the heads off in order to show they came to trade. Not a lot of business would have been done otherwise.
And that, as far as I can determine, was the sole reason for the vertical stem and stern posts.
Etymologically, according to
Wikionary (Old Norse, Etymology 3),
beit is, among other things (primarily to do with cutting or splitting), an old Norse word for
boat, and, in an apparently related context,
beitiass is a sail-yard or yardarm.