It's really just a management concept. How would you like to manage the devices or see them?
If you have a single IT team for your company, then a single network might be fine.
If you have a separate IT team per country, and you want each country to manage their own devices, then you might have a systems manager network per country.
Or you might like to segregate them by function to make creating policies easier. Perhaps one network for BYOD and another for company-owned devices (assuming they have very different policy requirements).