QA is “the system should do this; does it do it?”
Dev is “the system should do this; make it do it.”
I’m not putting one above the other as both functions are needed for most successful companies. I am saying they’re not the same job.
It’s already assumed that the product has certain features. And that those features work a certain way. But what if you did something else, that was unexpected. Then does it break the system?
As a QA Engineer, your job is to find that fault, and break it. Because you are possibly the last line of defense, before that software goes out into the real world.
More seriously there is a lot of differences between the two tracks but a QA person who can write clear and repeatable reproduction steps will save a company a whole bunch of dev hours and doing so usually requires a pretty good understanding of why the system is actually breaking.