King John evidently does not want Robin Hood to speak. Figuring the crowd wants to hear what Robin has to say, and since Robin is eager to speak, the King figures he may as well let him speak.
The expression "if you must" indicates a bit of impatience on the part of the one who says it. This person thinks there are better things to do than what you want to do, but he'll let you do your thing--again, only if you must. Notice, however, that in the case of Robin, the "must-ness" is on Robin's part, not the King's. If the King were to say, "You must not speak," then Robin would not have spoken. The King's words, in that case, are an order. Must-ness, as answerer Jasper Loy, above, suggests, indicates something that has to be done, in the opinion of the one who wants to do it.
Now if the person who wants to do or to say something thinks it can wait, he might say,
"Well, it's not really that important. I can wait until a later time,"
in which case, whatever needed to be said or done wasn't that important.
The word must denotes something urgent, but what you consider urgent and what another person (such as the King) thinks is urgent are not necessarily the same. If the person in charge thinks it's not urgent, he may still give his subordinate permission to speak, but only because he has the authority to grant permission.
In conclusion, perhaps an example might help. Keep in mind, the person who says "If you must" is a little impatient, and the person to whom it is said thinks what he has to say or do is urgent.
A husband and wife are having a serious discussion. All of a sudden, their 18-year-old daughter bursts into the room and says, "Mom. Dad. I have some really good news," to which the mother says to the daughter, "Can't you see your father and I are having a serious discussion?" to which the daughter says, "But Mom, it just can't wait!" The mother, a little annoyed, and with a sigh, says "Well, if you must tell us your good news, and it just can't wait, go ahead and tell us." The daughter then says excitedly, "Mom. Dad. I've just been accepted at Harvard!"