That answered the need for beaching them in some fashion but why the roofs. Ah ha, thee hot summer sun promoting mold etc. Thus the cover, much like the ubiquitous blue tarps at modern marinas. Video mentions hull plank shrinkage at joints that most likely required required re-caulking, but that's always been SOP for wooden ships.
Optimum hull shape for speed IIRC, is light, long, slender, and shallow draft, like racing sculls and Viking like boats.
Always wondered about the triremes efficiency, where stroke timing must be precise. Came to the conclusion that the sudden burst of ramming speed and mass gave them the ship killing power.
The video also provides a full count of the boat sheds as distributed between the three protected harbors, illustrated by the drawings. Impressive concentrated and coordinated effort by the fractious Greeks. In addition to the warning, Beware of Greeks bearing gifts, the adversaries added, Beware of Greek ship bearings. ;>)
If they were all under roofs, there must have been a lot more roofs, the Athenian navy was huge. Of course, a good many of the structures may not have survived, or been scavanged for building materials after the fleet was scrapped after Athens was defeated by those Spartan pederasts.