They don't do any favour. They are supposed to allow it by law because of GDPR. If GDPR was not passed, I personally doubt Apple would do anything. They should allow export not only to Google Photos but also for yourself to download in a way that you could import it to any other photo storage service.
How are they obligated by GDPR to support server side data transfer to Google in particular? My understanding is that they are only required to make it possible for you to download your data so that you can upload it elsewhere if you'd like to.
The right to data portability allows individuals to obtain and reuse their personal data for their own purposes across different services.
It allows them to move, copy or transfer personal data easily from one IT environment to another in a safe and secure way, without affecting its usability.
It doesn't say that they have to implement moving from one provider to another without you having to download the data, but I think the key phrase is "easily from one IT environment to another in a safe and secure way, without affecting its usability".
