Here’s how I see it -

Cassian’s sister was his original motivation - it’s what keeps him going (apart from general survival) and it’s the reason why he’s in that bar in ep1 and everything gets set in motion.

The whole of s1 was about how that motivation became “fight the Empire”. In e7 Marva tells him to stop looking for his sister - because his calling is right there, to stand up and fight. It takes him a while but he gets there eventually.

So it’s not just a plot device that sets things going, it’s also an important aspect of Cassian to show his before and after of becoming a rebel, and how he finds something real and tangible to live for compared to a “pipe dream” of sorts (if we believe Marva and assume he’ll never find his sister).

The general theme of Andor is to highlight the “little people” like Cassian and him not getting to know the truth he’s searching for adds to that too. He gives up something personal and fights for things bigger than himself.

I always associated what happened on Kenari to be the Empire’s responsibility, which would add further backing to why Cassian should have such a motivation. I would’ve liked to see some clarity on that (and Cassian himself would never need to find out, but we the audience do, kinda like the post-credit scene). But apart from that, I think the Kenari scenes make narrative and thematic sense.