Moving heaven and hell to retrieve lost comrades sends 2 messages. The first one, to your own forces, says we will not abandon you. This is why so many service members were upset by the way the Obama administration sat on its keisters and watched the Benghazi compound for 7 hours as they fought for their lives. You may not be able to get them but you can try. There's a scene in "A Bridge Too Far" that captures exactly this as the Brits look at the delays in moving their boats up the road so they can cross the river to rescue their comrades; in typical understated British fashion, one officer states "well that's it, there's nothing more we can do," to which the commander of the 101st AD says, "but try. You can do that, can't you?"

The second message sent is to your adversaries, we will not stop, we will not relent. The idea there is to tell the adversary they can not, will not wear you down. If they want an end, they need to negotiate in good faith. The latter condition will never happen with Hamas but we (Western nations) feel compelled to give them a chance to.