This is actually a very shrewd question. Let us remember that this play is about kingship, and in particular tries to show us the process by which Henry V becomes a "perfect" King, facing many different trials and challenges. We see him in Act I having to decide whether to wage war or not, having to handle traitors and generally facing the realities of kingship with all of the hard decisions that those realities involve. In this scene, we see a King who now has to rally his troops and encourage them after an unsuccessful attack, and somehow inspire them on to victory. Note how the speech starts and what this reveals about what has just happened:
Once more unto the breach, dear friends, once more;
Or close the wall up with our English dead.
The way in which this famous speech begins by saying "once more" to battle, indicates that this is not the first time that the English troops have tried to storm the castle. Likewise, the second line indicates that there have already been a number of casualties. We seem to be seeing Henry facing yet another crisis moment of his reign, where his troops are dispirited, lacking in motivation and are struggling against defeat. It is Henry's responsibility as their King to inspire and enthuse them so that they find the energy to win this important battle.