Unix time (which is UTC, and expressed in seconds) does not embed a time zone, which is useful information in some contexts.
For example, with e-mail you want to know both the absolute time (so the mail client can indicate how long ago that was) and the sender's local time (important for human interaction).
Implementation wise, making the timezone explicit forces the implementer to think about it at least once and not make the mistake of writing a local time, which works while testing on systems that are on the same timezone and breaks down in the wild.
For example, with e-mail you want to know both the absolute time (so the mail client can indicate how long ago that was) and the sender's local time (important for human interaction).
Implementation wise, making the timezone explicit forces the implementer to think about it at least once and not make the mistake of writing a local time, which works while testing on systems that are on the same timezone and breaks down in the wild.