I think if I was to have a sound designer on my own project, I would want some level of uniqueness to the sounds. Similar to art, I wouldn't want to hire an artist just to make art that looks like asset-store content, I'd want unique pieces that complement the feel of the game. Sound should be similar, I'd want the music and sound to tell a story, and to be something memorable to players. I'd want a sound designer that understands that, and could either choose specific sounds to match that, or to modify or create new sounds to achieve the goal.

I wouldn't want to hire a sound designer who simply chooses sounds from existing packs they own and puts them into the game. I'd want them to be able to make new sounds if needed, or modify existing sound effects to make them unique in some places.

(I have worked on some projects where the sound person simply made some lists of pre-existing sounds/music tracks and asked which ones we liked.. this is really not the kind of choice I want to make as primarily a programmer. I want a sound person who can decide what they think sounds best for the game),..