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 

How do we know the Vikings were here?

The Vikings have left many traces of their settlement which are still visible today. Archaeology provides evidence of their adventures, settlement and their daily life. The study of their 
language shows the lasting effect that the Viking settlements had in the British Isles, and DNA analysis shows the effect the Vikings had on the genetics of the countries where they settled. All of this gives valuable information, but the only reason we have non idea of the 'Vikings' as people is their appearance in the drawings found by archaeologists.
http://www.dightonrock.com/thevikingsneverhadanaythingtodow.htm

The first findings of Viking artifacts were found in the early 1060's. The first recorded raid on Britain was in 789 (three ships raided Portland). The last raid attempt was Harald Hardrada's in 1066 which failed spectacularly. 

Archaeologist Patricia Sutherland (orange jacket) excavates a potential Viking site on Baffin Island.