This simile suggests a lot about both the soldiers and the war that is leading them to be killed in such a spectacularly numerous way. The key to understanding any simile is thinking about the kind of comparison that is asserted and what that shows and says about the first object. Let us just remind ourselves of this simile and the text that surrounds it:
What passing-bells for these who die as cattle?
Only the monstrous anger of the guns.
Only the stuttering rifles' rapid rattle
Can patter out their hasty orisons.
The bitter anger of Owen's words is obvious when we see how the poem develops and he protests at the lack of any passing remembrance for the many soldiers that die. In many ways the simile is shocking. In our minds, we do not associate soldiers with mindless cattle that unprotestingly drift towards the slaughterhouse to die with perfect equilibrium. Being described as "cattle" suggests a dehumanisation of soldiers that does not identify them as humans. This is of course precisely Owen's criticism of those who are in control of them. War is the ultimate dehumanisation of man, and this simile helps to express this message.