La Stella hits lefty, is pretty limited defensively and is coming off a season in which he hit .266/.340/.331 (78 OPS+). He's not really gonna hit for power, but the career on-base percentage is .345 and he did hit .288 in 2017. One area he's been good is as a pinch hitter. He's a career .278/.394/.386 hitter with 22 RBI in 192 career pinch-hit appearances. That doesn't quite carry as much weight in the AL, but some might find it interesting.
La Stella's likely fit on the Angels, as things currently stand, would be either as a bench bat or a platoon-mate with David Fletcher at second base. It's a long offseason and lots of things will change, though.
More interesting here is the Cubs swapping out La Stella for Torreyes. The latter provides the capability to play shortstop, which La Stella does not. With that in mind, the eyes turn to Addison Russell, who is serving a domestic violence suspension that continues into 2019. Perhaps Torreyes serves as the backup shortstop to Javier Baez until Russell's suspension is over or perhaps the situation is for the whole season while the Cubs either non-tender Russell (the deadline is Friday) or trade him at some point.
Regardless, this swap has Russell's name all over it, otherwise there wouldn't have been great reason to rearrange the infield furniture like that.