The rationale is easy to follow. Evidence that goes into the jury room without the knowledge of the court hasn't had the benefit of being tested. It might be inadmissible, or prejudicial, or capable of an explanation by one of the parties, who didn't think to present the explanation because it didn't know that the jury had received that information. Jurors are repeatedly told in the course of their service that they must make their findings solely on the evidence presented to them.
The prohibition on research most often relates to the facts of the case, and there are numerous examples of individual jurors (or more rarely, entire juries) being discharged after contact between juror and party, either before or during a case.
In rarer examples, jurors have been discovered conducting their own research into the case. Notoriously in R v Young  QB 324, a retrial was ordered after it was discovered that some of the jurors in a double-murder trial had used a oiuja board in an attempt to contact the victims.
(Fortunately for Henry Fonda, the judge never found out what he'd been up to):
In Martin v The Queen  VSCA 152 the Court of Appeal was confronted with a different problem. After the verdict of the jury had been given (guilty, obviously, given that it was the accused who appealed) the judge's tipstaff found 7 pages of material downloaded from the internet, discarded in the jury room. The pages concerned the legal definition of the phrase beyond reasonable doubt (a phrase which the High Court has notoriously admonished trial judges not to attempt to define for a jury, even should they ask). It raised the question, should a jury be prevented from conducting its own legal research?
The question didn't really get answered, as the Court felt that to consider the impact that the material may have had on deliberations would either amount to baseless speculation, or else intrude into the confidential workings of the jury room. Ashley JA referred to R v Chatzidimitriou (2000) 1 VR 493 where a trial judge allowed a jury access to a standard English dictionary to assist them in deciding the meaning of the phrase.
73 The question which I posed in the preceding paragraph needs to be considered, in my opinion, in the context of the standard of proof direction; not in the context of a general enquiry about the permissibility or otherwise of jurors researching the legal principles applicable to the case before them. The authorities make it very plain that it is for the jurors to give meaning to the critical phrase. The fact that a judge may not elaborate upon its meaning, except in particular circumstances, does not mean that jurors may not consider what meaning to give it.
The Court of Appeal advised trial judges to tell jurors that s 78A extends to searching legal dictionaries (though the Court did not actually find that this was the case). If such a direction had been given in this case it would have been easier for the Court to find that an irregularity had occurred (the jury would have been in breach of the trial judge's specific instruction). But, of course, this provides no assistance to jurors in determining what the phrase should mean.
For some observers Martin's case no doubt adds to Vincent J's criticism of the insistence in Australian law that beyond reasonable doubt not be further defined. (He described it as 'ridiculous' [at page 40] in his report to Parliament regarding the conviction of Farah Jama by flawed DNA evidence). If jurors are left to search the internet for the meaning of terms which are central to their decisions, there's a strong suggestion that jurors are not being properly assisted in their task. It may, but then again may not, run contrary to the current popular opinion that holds that most jury charges are overly long, needlessly complex and full of irrelevent material.
Against this backdrop, the state government has coincidentally announced plans to widen the jury pool to include people who are currently excluded from service. Last Tuesday, Rob Hulls announced that lawyers, judicial officers, police officers and Members of Parliament will be able to serve on juries sooner after leaving their office or practice under reforms to Victoria’s jury system.
The prohibition of people intimately involved in the day-to-day operation of the courts traditionally doesn't just stem from a concern that they won't be able to shed any previous bias to one side of the adversarial system or the other to perform their role as a juror. There was also a concern that other jurors might be inappropriately swayed by a juror with intimate knowledge of the justice system and not go about reaching their own verdict independently.
Given the lack of guidance being provided outside the jury room, it's interesting (and, in some cases, perhaps worrying) to think about the potential influence that a retired judge, police officer or practitioner might have.