Mac Williamson has been dreadful since returning from his concussion. The Giants probably figured that was a possibility, and to their credit, they gave him a long leash to see if he could work his way through it.
I posted this last night:
So, this makes sense. The Giants brought up Slater because they could still use Williamson’s option to prevent a transaction mess. They’re clearly not ready to make any big decisions with Hunter Pence and Austin Jackson, and they seem intrigued by Alen Hanson in left field, so far now the 4th/5th outfielder logjam remains, just with a couple of different faces.