Trial by Jury?

According to the New York Post a Judge has ruled (case is irrelevant) that a jury should be informed of mandatory sentencing.

What is the core purpose of a jury? The concept of a jury seems to have begun in an attempt to remove the appearance of conflict of interest between those in power and the people they preside over. Traditionally a jury is a group of people compelled to render a verdict as to the guilt or innocence of one of their peers. They are not generally allowed to interpret matters of law, since those are handled by the judge. If the judge decides that the jury is ruling contrary to clear evidence in the case, the judge can declare a mistrial and call a new jury. The judge also decides which information the jury will have access to. This has the potential to either preserve the impartiality of the jury or to skew the opinion, depending on what was withheld. The system seems to work, more or less, but it does bring to mind the saying that if voting could really change anything, it would be illegal.

If the job of the jury is simply to decide whether the defendant committed the crime or not, why would they need to know what sentence they could be condemning them to? I see three reasons this should be allowed. For one, if the true purpose of the jury is to see to it that those in power treat their peers fairly, sentencing seems quite relevant. Secondly, if a member of the jury is already familiar with the law, they would already know the answer, so why withhold it from the others? And finally, sentencing is not evidence directly relating to the case, so I see no reason to justify withholding it.