This answer may be mostly relevant to the US. You asked about the general education goal and research.
Most US programs require the candidate for a degree pass a set of comprehensive exams in their field. There is advanced coursework that makes passing these exams possible and may be required or not. These courses give a broad view of the field making it possible for a future academic to teach in several areas, not only the narrow field of their research. Most US academics, even in R1 universities also teach.
The research goal, as you suggest is to give the candidate experience in plunging deeply into one (usually) area and to produce publishable research and extend the state of knowledge in the field.
In some fields, perhaps many lab sciences, it is possible to publish early in doctoral study, though not likely sole authored papers. It is possible to contribute to a thread of research early on without actually leading it in those fields. In other fields, say theoretical math, that is much less likely and is serendipity when it happens. Many students will finish their degree with no published works and only one or two having even been submitted for publication.
Hiring, when it is possible, depends on promise shown by a candidate and attested to by advisors and other faculty. Currently, recent graduates normally go through a postdoc period, but that is, I think, mostly due to economic constraints, not design. It is a reaction to having too many candidates for the available jobs.
The tenure system is intended to be the bridge. If you get hired into a tenurable position then you still have six or so years to show that your promise is also fulfilled. It is then that publishing becomes a vital concern.
In some fields, such as CS, where conferences are very important and frequent, many doctoral students become known generally to hiring institutions through participation in workshops and such (poster sessions...) though they may not have publications. Their work towards such may already be known to people who might hire them even before they finish.
But, there is more to an academic career than research. And many new PhDs will want a mostly teaching career in any case and the expectations of new faculty at Liberal Arts Colleges is quite different from those of R1 (research) universities.
I believe that we are currently in a period of imbalance. This is partly, though not entirely, due to COVID. At some future point the "requirement" to do postdocs may lessen. I hope so, anyway. It would bring more security to young academics than the current (lack of?) system.
An alternative idea, is that a doctoral program serves, intentionally, the needs of those people who are just driven (driven, I say) to learn the deep secrets of a field. This is pretty common in math, for example, and some others; physics, philosophy, ... Sort of like Hogwarts in some respects.
