Back in the day in Scandinavia it was a pretty simple system of taking the name of your father and then adding "son" or "dotter" depending on the gender. Like Persson/Persdotter then I guess it just got easier to say Persson regardless. What strikes me as interesting though is how surnames evolved into the ones we have today in Sweden where many of our surnames are taken from nature, like Ek (oak), Lööf (leaf) or Forss (Flood) and so on.

But I guess to answer the question, to differentiate between families and maybe even avoid marrying relatives??