Adding a user in the Admin Console creates the account, but a handful of related settings determine whether that user actually has a working, correctly configured mailbox from day one.
Licensing
Each user needs an active license assigned before their mailbox is fully functional. It is easy to create a user and forget this step, which results in a login that works but Gmail features that do not.
Organizational units
Which organizational unit a user is placed in determines which policies apply to them — sharing settings, security requirements, app access. Placing every user in the same top-level unit by default means you lose the ability to apply different policies to different teams later without reorganizing everyone.
Aliases versus separate accounts
If someone needs to receive mail at more than one address (for example, a name change or a departmental address), an alias on the existing account is almost always the right choice over creating a second full account — aliases share the same inbox and do not require a second license.