Giannandrea is a machine-learning expert who joined Google in 2010. According to Engadget, he will head up the "machine learning strategy and AI" and will be directly responsible to CEO Tim Cook.
It remains to be seen whether Apple has a specific plan for Giannandrea, but Cook mentions in a message sent to employees that Giannandrea has "a similar commitment to our privacy and our approach."
In other words, Cook will not begin to mine user data for Giannandrea just because he has worked for Google.
When Giannandrea resigned, at first, he mentioned will still work at Google. However, he seems to change this decision. Another possibility is from the beginning, he did want to move to Apple.
Whatever the reason, Giannandrea has an important role in Google in the past few years. He was the one who pushed AI technology into the center of the Google product.
Google Assistant is on many devices, and machine learning technology can be used in many ways, from mobile photography to image search.
This Apple decision shows their seriousness to improve their quality. Apple was the first company to introduce virtual assistants on smartphones with Siri.
However, Siri's popularity was quickly outclassed by other virtual assistants made by Amazon, Google and Microsoft. Problem answered the question, Siri is not as good as a virtual assistant made by Apple's competitors.
Giannandrea may not be able to instantly make Siri as good as Assistant considering Google also has its own search engine but at least he can ensure Siri is not inferior to its competitors.