To Remind you...this was when they got Married
This is of course her main Royal Duty...for She can NEVER rule. Though she will be bestowed with the Title of Queen, most probably...she is what is known as "Queen Dowager" NOT "Queen Regnant" in theory we have a King right now...The Duke...He is King Dowager.
If they feel its inappropriate they can use a number of terms...for example, they could call Catherine, in future, The Kings Consort
But there has only ever been ONE time where BOTH Male and Female Married Monarchs were both Regnant and Soverign...and that was Queen Mary, and King William. They were Co-Regnant during the time of The Glorious Revolution.
Therefore, he main Royal Duty is NOT to lead...but to Produce the continuation of the Royal Line. What she has to do is Breed, successfully, and where possible produce Male offspring
and they like to start as soon as possible, because obviously these Babies have to grow up before they can reign in their own right...so you get married, and then have lots of children...and hope they are nice and ripe in age before they inherit the throne. This ensures not only the Monarchic Future, and the line, but more importantly, it provides a future for the Ruling Dynastic House.
In past times, failure of the women on this level has meant that, if they are monarchs their houses are doomed to die with them, just like what happened when Elizabeth 1st died...and if they are not Regnant...well..usually they got discarded at best...murdered at worst.