iis7/8 with web/ntfs authority question?

environment： win8.1，IIS 8
 
I have deleted all accounts on the site directory，only set IUSR(NTFS read).
the client access the site, there is a page that need to create a new html page in the web site, but access is refused.
So I set the IUSR( NTFS read, write ) permissions on the site directory  , but was refused.

I have tried to add the w3wp.exe's account AppPoolId on the site, set NTFS read,write.
The result can be run correctly, but if i removed the IUSR account, the result cannot access the site.
If the website is subject to the IUSR and AppPoolId effects of the two accounts together.
 
I don't understand, the client request using the IUSR account, Why add AppPoolId account?