Coining money in Viking Age and more details on found hoards
Consider there was no secure way to store wealth in the time of the Vikings. There were no banks, no marketable securities, no way to establish reliable title to land. In essence, there was no store of value other than bullion or coins.
So, if you had a huge amount of wealth beyond your livestock and weapons, it would be in the form of bullion or coins. To keep it safe from theft, you would have to hide it in the ground.
Wealthy people who were killed, captured, never came back from another raid or trading expedition, or merely forgot where they buried their wealth abandoned all of their underground accumulation to be found some later time.
In The Age of the Vikings, Anders Winroth says children on a school outing found one hoard of 1,452 silver coins from the Viking age. Almost the entire hoard was Arabic dirham coins.
Author says there have been over 700 hoards found in Gotland. There was so much silver buried that another find is usually discovered each year.