There are several ways that this would benefit both Google and Wikidata. First, we currently extract a lot of data from WP infoboxes and load that data into Freebase which eventually makes its way into the Knowledge Graph so linking the two datasets would make it easier for us to extract similar data from WikiData in the future. Many other tech companies and researchers are doing similar extraction projects from WP and Freebase so this would benefit them as well.
Secondly, we'd like to contribute (or enable the Wikidata community to contribute) the data that we've already extracted from WP infoboxes back to WikiData. I'm not quite sure what the best way to do that is (pull by the community is probably better than push by Google) but having the linkages between equivalent concepts is an important first step to sharing more data.
Lastly, the Freebase community does a lot of work to clean up data that was imported from WP, OpenLibrary, MusicBrainz, etc. including merging duplicate topics and splitting apart conflated topics. This is important but tedious work that often doesn't get pushed back to the original sources for no other reason than there simply isn't a well-defined process for how that should work. I'm hopeful that the WikiData community will find a way to benefit from the cleanup that we do in Freebase creating a virtuous cycle that improves the quality of both datasets.