This act takes place in the camp of the Gascony Guards at the siege of Arras. The soldiers are all suffering from hunger, for while the French are besieging Arras, the Spanish have encircled them and no supplies can be brought to them through the lines. Cyrano, at great risk to his life, has found a way to get across the lines and he does so in order to send "Christian's" letters to Roxane. The reason that he does not bring food on any of these trips is that it would be too bulky for him to carry and still be able to evade the Spaniards. Cyrano says that he thinks there must be a change soon, that the company will either eat or die: the Spanish are planning something.
Act III has been a light, often humorous, act. Now, however, the mood undergoes a very definite change. This scene sets that mood by showing us the state of the war and indicating that the situation at Arras is very serious. The atmosphere of gloom deepens throughout the act, with only one touch of lightness.
How typical that Cyrano's dangerous journeys through the enemy lines are made for spiritual and not physical reasons! Keeping the promise of frequent letters, which he made for Christian in Act III, Scene 12, does not seem sufficient justification, especially because he cannot bring food back with him. (And we can be sure that, with his friends approaching starvation on the other side of the lines, he would not avail himself of the opportunity to eat.) Perhaps he feels that once the war is over he will never have another chance to tell Roxane of his love, and he wants to do that more than anything else, even if he must sign Christian's name to his own letters.