You do a clean-room implementation to preserve the ability to say "even though these lines of code are similar, they couldn't have been copied because we locked these engineers in a room while they were writing this implementation." But it's also part of the story: "we're good guys; we did all the work ourselves from scratch instead of trying to leverage all the work our competitor already did."

Which is part of the whole rationale of unfair competition law. It hits a moral button when someone takes advantage of someone else's work to make themselves a bunch of money. The more intentional and purposeful that copying is, the more wrong it seems.

The dynamic that requires companies to clean room, typically comes from some license that comes with the original product.

I license you a black box with 3 switches on it and 3 lights. The license dictates you may not look inside. The clean room implementation is, I right down what the effect of each switch has upon the lights. I then give that specification to an engineer who's never seen the box. They build a copy of the box, based on the spec.