Two things it solves: You get to evaluate me, my ability to deliver, and how I interact with your team and I either bring real value within two weeks, or I don't. I can tell you verbally I am an indispensable asset or I can show you; other people have ruined the verbal trust layer which is why this whole debacle exists in the first place btw.

And more importantly, but less communicated, I get to evaluate you. How your team works, the level of talent present, management's ability to keep direction, and wether I genuinely enjoy what you have for me to work on.