The comparisons you refer to are notable for the way that they deliberately attempt to present war in an incredibly brutal and realistic way. Given the title, and its presentation of soldiers, we automatically summon up an image of smart soldiers in their uniform marching together and bravely fighting. The image in line 1 therefore deliberately contradicts our expectations by presents the soldiers as being "like old beggars under sacks." There are no smart, young, brave soldiers here, but only men who have been prematurely aged by their experiences.
The image in line 14, when the speaker sees his fellow soldier "drowning" through the gas mask likewise challenges our expectations of soldiers. We expect soldiers to meet their deaths in battle fighting against the enemy, but here we see the speaker's friend dying ignomoniously away from the from the front line by a gas attack, and "drowning" rather than dying in hand to hand combat.
In line 20, we are given an incredibly grim image of the corpse as "the hanging face" is described as being "like a devil's sick of sin." No noble, glorious death for this soldier, only a death that horrendously disfigures his body with his sufferings.
Lastly lines 23-24 emphasise the pain and agony that this dead soldier endured by focusing on how terrible his death was:
Obscene as cancer, bitter as the cud
Of vile, incurable sores on innocent tongues--
All of these images are therefore alike in the way that they profoundly challenge our ideas and expectations of soldiers and battle, presenting it as a terrible, dehumanising and demeaning experience involving tremendous suffering and no glory whatsoever. This of course supports the main message of this excellent poem.