[janef0421]

If 'Oumuamua is a probe, it appears that it would be a fairly simple one. To a sufficiently advanced civilisation, it would probably be relatively low-cost. If an alien civilisation has a reasonable degree of curiosity, sending a cheap probe to examine another star system is likely worthwhile.

[SuoDuanDao]

I would imagine there's also a universal drive of life to try and spread. a probe that can parachute microbes to earthlike planets as it 'buzzes' them would seem worthwhile on its face to me. I rationalize it as a way to 'pay forward' the debt we owe for the current mass extinction, but on some level it's probably also an abstraction of my sex drive.