They'll know what they want but not where or how they want it. It might just be "a sink" and the competent designer will find out about their usage patterns to

They'll know what they want but not where or how they want it. It might just be "a sink" and the competent designer will find out about their usage patterns to know number of sinks, size, style, colour, depth, faucet type -- and have it work within the constraints of the space. ...while also aware of the macro organization of the space itself.