Azure AD MFA is not enabled by default for AAD and Microsoft 365 users, but it will be if during setup an admin chooses to Enable Security Defaults on Azure AD (as most will when prompted to do so: after all, who in their right mind wouldn't require MFA?).
However, there are situations where being able to toggle MFA on or off for a particular user can be useful. For example, in a development or test tenant when working with sample code from Microsoft that doesn't account for MFA (like a simple Microsoft Graph console app). This procedure involves disabling Security Defaults on AAD.