Hi mr karaoke,
Our bank requires the interim OC before releasing the final progress payment. This is why KH make he final payment $2000, so you can basically pay it yourself then get your keys. Otherwise if the bank needs the interim OC to release the final sum close the construction phase then you'd be stuck waiting a few more weeks before getting it.
I still don't have an interim OC, because I still need to do retaining walls and steps at the afresco. I found this out one week after I got my keys!
Not all banks require interim OC for final PPA, in my case they did, so I guess KH has built this into their payment schedule to cover this scenario.
Hope this helps mate !