a.) He's convincingly religious, and could hopefully pull some moderates or Republicans who were disgusted by Trump but afraid of voting for a closet atheist.

b.) He speaks good Spanish -- a crass political point, but good for juicing Hispanic turnout particularly against Trump, and developing long-term party loyalty.

No idea if either of these points manifested in the race, but they seem to constitute a reasonable argument.

For the record, I agree that Sherrod would have been a better choice, particularly in hindsight.