Update: I was wondering whether there was any insight into looting when I wrote this post. Probably only minor similarities.
The Anglo-Saxons raided on the English coasts for years before they started staying over the winters and eventually conquering territory and setting up farms of their own. They got their foot in being hired by the leftover nobility to protect those coasts against other raiders. It is a common story, which the Romans used often enough that by the fall of the Western Empire, most of the Roman army was made up largely of those tribes which had been doing the invading, such as the Goths. In England, Ireland, and Scotland, the pickings were pretty easy, as there was no unified response and the invaders had much greater mobility. Over a period of two centuries, the invaders gradually became neighbors and trading partners, more in some places than others, but still consistently.
A couple of centuries later the Vikings - both Danish and Norwegian and thus not always allies of each other - did the same thing, starting off by raiding. Easy pickings again, as they were able to smash and grab, making off with gold and silver, only later trying to permanently take territory and settle in as farmers. Pillaging was a precursor to conquest, and the original pillagers did not tend to think of themselves as an advance guard for conquest. They just had this job, pillaging with low risk, and set about to do that. They had somewhat less impact on settlement overall, though they established strong y-chromosome dominance in substantial areas, indicating that they displaced the original males.
Well, raiding is easier than governing, if one is willing to take risks. This was all before the long process of reducing intragroup violence that began in NW Europe and eastern England around 1100 with a) feudalism or b) avoiding cousin marriage, so enormous cruelty and violence were still the reality. The English also used the strategy of buying off the invaders with ever-increasing sums of money, with the hopes that they would go home, or at least go Somewhere Else. Sometimes it worked, sometimes not. Usually the latter. They found they had to fight eventually, except now thy had tens of thousands of pounds less money with which to raise an army.
If you wonder how they could be so short-sighted, not seeing the danger to their country in this, it was because these were the rulers doing the paying off, and their more pressing need was usually someone else who wanted to be king, not the general welfare.