Perhaps the intent is to not be belittling, or, relatedly, not imply that something should be trivial for everyone.
If someone replied to me that I "just need to set up the Kafka consumer" I'd have no idea how to do that, and would potentially feel embarrassed about needing to ask for further information.
Yeah you’re right. Most sentences don’t survive subtracting the word “just” exactly as they were written. It’s more food for thought. I agree it changes the meaning, and I wouldn’t advocate a universal ban (your example is evidence for the second half of my original post, I think).
You're not just pruning, but changing the meaning. The latter implies there is only one thing you need to do, while the former is open ended, and could imply the beginning of a set of needs.
"Just" can also add emphasis. "That's stupid" and "that's just stupid" have different nuance.