Inside our Office 365 tenant's Domains page >> we have the option to add additional domains beside the default domain as follow:

So what is the purpose of this step? will our Office 365 tenant have 2 separate domains?
Inside our Office 365 tenant's Domains page >> we have the option to add additional domains beside the default domain as follow:

So what is the purpose of this step? will our Office 365 tenant have 2 separate domains?
The purpose is to be able to use domains you (your organization) owns for things like email, chats/meetings, etc. It's not mandatory, as you get a default domain with O365, but it's something that pretty much every business out there does. Back in the day you were even able to host a webpage with your custom domain name in O365.
Hi johnjohn-0472,
Agree with michev. Normally {tenantName}.onmicrosoft.com is the default domain of Office 365. You could create accounts specific to one domain by adding your own domain.
For Reference:
https://practical365.com/configure-a-custom-domain-in-office-365/
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