The Brewers designated Bill Hall for assignment last week, so they had to do something with him. Hall has a ton of money left on his contract, and presumably Milwaukee will pick up a ton of it in the trade. Otherwise, this deal doesn't make sense for the Mariners.
Hall has been a mess at the plate this year, and I don't think he's a strong candidate to bounce back all that much. He has power, but strikes out a ton. That's always been his hitting style, and ever since his breakout 2006 campaign, his numbers have declined dramatically. It looks to me like he was lucky in 2006, and on top of that pitchers figured out how to avoid his power better, while perhaps Hall has tried to muscle up a little too much for his own good. Now he's coming to the AL, which is a stronger league than the NL.
As a bench player though, I like Bill Hall. He's been a good defender throughout his career at several positions, most notably third base. His ability to play multiple positions well, and threat to hit the ball out of the ballpark, makes him a good fit for the M's needs right now.
Hall has hit lefties better than righties in his career. A platoon with Jack Hannahan at third while Beltre is out would make sense, and pinch-hitting for guys like Michael Saunders and even Josh Wilson (though Hall may take his roster spot) would make sense too.
Jack Zduriencik saw Hall at his best while both were in Milwaukee. Hopefully, Z sees something that can be fixed, not just visions of the past with hopes Hall will magically find it again. Even if Hall doesn't hit much, he's still a worthwhile addition.