By "the Heptarchy" I mean the petty kingdoms in the British isles around the time period of 500-800. At the time Britain was divided into about a dozen little kingdoms that often waged war on eachother. However, none of them were strong enough to unite the whole island or, or often even to conquer their neighbours. It's often stated that the wargoals of these conflicts weren't to outright destroy or annex enemy kingdoms, but to "dominate" them. I have trouble wrapping my head around what that concept means. Perhaps it's because I'm used to modern wars being about annexing territory or "uniting" an area under one's own state. However, the British and Anglo-Saxon kingdoms often won wars without any changes to the borders. What exactly were they fighting about then? What was the goal of the wars?

Bonus question: I've read somewhere that there was a custom among some of the kingdoms called the "king's raid" or some such, supposedly it was expected of new kings to attack a neighbouring kingdom upon their accession to the throne, as a tradition. Is there any truth to that?

Thanks in advance for answering my questions!