When the vikings plundered monasteries, why did they value decorative objects of admittedly precious metal?

Answer 1

Because they were easy to carry and could fetch a good price om terms of food and supplies.

Answer 2

Because they could melt precious metals (gold, silver) down. Then the chieftain could order the metal forged into rings, coins, and other bulk pieces. The chieftains could give the objects to their followers. In old Viking sagas, a victorious chief was praised by poets (skalds) as "Gold-giver!" "Ring scatterer!" It is a feature of modern English, which we inherit from Old Norse poetry, that we try to find as many ways possible to say the same thing. When the Norse chieftain kills his enemies, "He feeds the ravens!" "He battens the wolves!"