If I remember my early medieval history right most longboats and their relatives were set up so that the mast could be quickly set up and taken down to meet the needs of the moment. For instance if the ship was maneuvering in to battle (allegedly vikings would lash their ships together and sort of crash in to the enemy formation, then fight from deck to deck as though they were on land! idk if it's true but…) they'd take the mast down so it wasn't in the way. The mast could also be unstepped for storage or for better maneuverability on a river where there might be trees or other overhangs.
I'd be neat in Valheim if we could unstep and stow the mast to make it easier to navigate under bridges or other overhead onstructions.
And while we're on the topic, I wish we could drag the ships ashore for storage. IIRC Vikings mostly didn't build boathouses the way the Valheim community loves to do and longships were mostly stored by dragging them up the beach and sometimes inverting them. I believe in winter they'd store them in long, low sheds.
Source: https://www.reddit.com/r/valheim/comments/101bbyz/itd_be_neat_if_we_could_unstep_the_mast_on_the/