If we press “Add Page” modal within admin plugin > it shows page creation modal, after we fill info, we need to click save again in order to use media field, what purpose of this?
Hi, did you get anywhere with this? I’m looking for the same answer.
Thanks
Most obviously it is because the page at first is just created in the virtual world, and not saved on disk. After your ‘second save’, which is actually the first save to disk, the hooks like media can start doing their work.
You could look at it as a mother and a child,
If there’s no mother there can be no child 
The child needs the mother before it can get born
As soon as the mother is there to give life, there’s a possibility for the child to get linked to the mother.
In a relation point of view,
There can be no childs if there is no parent available to link a child with
Hope it helps ya!