We need to know how they reached their decision so that those who have been wrongly convicted have recourse to challenge their decision. In many cases, innocent defendants end up in jail for long periods of time without ever knowing how they got there. There is so much wrong with the system before we get to the jury's decision, but at least in the end the defendant at least should know what made the jury decide against him. It could quite possibly be used as part of the appeal process. This would make the process more open and fairer.
Juries make life altering decisions, sometimes to the extreme detriment of defendants, especially innocent defendants. In every guilty verdict that is passed by the jury,we need to know how and why they reached that verdict.