I'd also point out that sailing cruiser squadrons around to steal stuff from the comparatively poor locals is a pithy but fairly accurate gloss on most of colonialism. You know, the period of time that space opera generally mimics, indeed the period of time that gives us the very notion of cruiser as a type of warship, which is in no small part a consequence of colonial activities. You need a long range ship to exploit the periphery, the gains of that exploitation are vulnerable to, and must be protected by, other long range warships. Ships that can cruise for extended periods of time. Cruisers.