Vikings named their swords because they were a status symbol and carried religious connotations. Swords became part of the Vikings' family heritage and were passed down through generations.
The Viking culture required all men to be armed. The most common weapon was a spear. Swords were expensive and, therefore, a status symbol of wealth. Vikings named them as one would name a member of the family.
Swords also carried religious significance to the Vikings. The only way to Valhalla, the Viking paradise, was to die in battle. The sword was, in essence, the key to Valhalla. The Vikings believed that naming it would give the sword the power of an ancestor or spirit animal. A family sword would carry the strength of all members of the family with it, giving courage and strength to the wielder.
The cost and spiritual connection to the sword made it valuable. It was financially valuable, but perhaps more so it was spiritually valuable. Anything of such value deserves a name.