Why did the Vikings used coins instead of bartering?

The Viking Age saw major changes in the economy of Scandinavia. At the beginning of the Viking age, very few people in Scandinavia had any knowledge of coinage. Some foreign coins entered the region because of trading contacts both with western Europe and the Islamic world to the east. However, except in major trading centres such as Hedeby and Ribe, in Denmark, the idea of coinage was unfamiliar. Coins were valued only for their weight in gold or silver, and circulated with many other forms of precious metals.
This is what is known as a bullion economy, in which the weight and the purity of the precious metal is what's important, not what form the metal takes. Far and away the most common metal in the economy was silver, although gold was also used. Silver circulated in the form of bars, or ingots, as well as in the form of jewellery and ornaments. For the Vikings, it was easier for them to carry coins because they migrated many times so instead of bartering for goods they made coins and used those.

These are some Viking coins found in Furness, North West England

These coins were also part of the find in Furness, North West England