Sami people
The main purpose of the birkarl organization was to control the trade with Sami people and tax them. Legends told that birkarls rights to tax Sami people was given by Magnus the Barnlock, the King of Sweden at that time. Sami people were traditionally taxed by Norwegians already in the Viking Age or even earlier. Later Russians started to tax them as well. After having southern Finland under control around 1250, Sweden became interested in the situation in the north. Eventually, some Sami people paid taxes to all three states. Birkarls were just one element in the colonial system profiting off of the Sami area.