A good QA person/team will (be strongly incentivized to) make it their mission to find bugs and usability issues. Likely they will be domain experts, if only in the domain of the usage of your product. Worth their weight in gold, no machine can give you that.

I've heard of good experiences using a 3rd party QA company (for frontend-heavy changes) and had pretty OK experiences doing it in-house with subject experts (though testing backend changes, so pretty much "either it works or it doesn't" in my case).