APS-C crop mode?

TLDR: what are the advantages of using APS-crop mode?

Now that the A7RVI dropped, I’m looking into crop mode, as this leaves about 30MP of resolution when used. I read a lot of different ‘facts’ and I’m wondering what truly is the benefit of using it for stills and for video.

As I understood: it only reads out the center part of the sensor and for stills it shouldn’t matter in terms of resolution if you just take a FF photo and crop it to the same FOV as the APS-C crop mode photo. It will give you more freedom to crop in post.
But I also read that the read out speed would potentially be faster, since the electronic shutter sync speed is higher in crop mode compared to FF mode (at least for the A1). This in particular interests me as I will shoot wildlife with this camera.

Is this indeed the case? Also, are there any other benefits as smaller files, more accurate AF and so on?

For video I noticed it will allow you to get a big crop in 4K120, which for wildlife would be nice! I also noticed that the 4K60 was oversampled from 6.3K. Are there any other benefits for video as well?

Thanks in advance!