Yes, it is mandatory to use the COA. It is used for two things:
- For OEM partners the app is for provisioning the underlying salesforce.com licenses as well as revenue sharing
- For ISVforce partners it is for revenue sharing.
Note that Salesforce explicitly tells you "Orders should be submitted based on the sales and licensing of applications to customers, as required by your partner agreement."
That means that you can manage the app's licenses through the LMA/LMO quickly (and you decide yourselves how your client pays you), but you are contractually required to also pay for those licenses to Salesforce through the COA, for the full contract period (which usually starts when you add licneses in the LMA/LMO).
If you are developing OEM applications, the purchase via the COA will actually provision the salesforce platform licenses to the specified org, so that your app can be used by actual users.
- App licenses are managed through the LMA/LMA. The revenue split part needs to be paid to Salesforce via the OCA.
- Salesforce Platform licenses that come with an OEM application are only provisioned once you complete the actual purchase via the OCA. Of course, your client could already use your app via a Trial, or even use more seats (that you added through the LMA) on their existing Salesforce licenses, but they will only receive the Salesforce Platform licenses once the order via the OCA is complete.
Hope that helps!