For a similar type of weapon, see Banner
Polestaves are single or two-handed weapons that are fully based on a crush attack style. They provide an attack style in each of the three melee styles, and do not provide any magic auto-casting. Among the polestaves, there are several types of banners such as the Western banners, and the three types of Mjolnir available from the Enchanted Key miniquest.
Polestaff weapons are essentially identical to staves and wands, except for the ability to auto-cast spells. Whereas staves and wands are able to select a spell to auto-cast in at least one spellbook, polestaves lack such ability and serve solely as a melee crush weapon.
Despite the mjolnirs being affiliated with the gods, they do not provide protection from their related factions in the God Wars Dungeon. In addition, they have stats roughly equivalent to that of an iron 2h sword. These reasons make the Mjolnirs serve no specific purpose whatsoever and gives them a mostly cosmetic purpose. Since players can only find each mjolnir once during the miniquest, players who lose or sell their mjolnirs can only re-obtain the item through trade with another player.
Other than the mjolnirs, most other polestaff weapons also mainly serve a cosmetic purpose, and are largely not intended for actual combat.