In-character, your characters can gauge the level of damage creatures have taken and may be able to withstand based off things like how bloodied they are, are they sluggish or not, and other common sense thing. Granted, zombies of course are difficult to gauge because rotting corpses, but at the same time you'd know that even the most fucked up zombie tend not to take violent and sudden dismemberment well.

Out-of-Character, you'll only get HP's if you roll initiative. I mostly keep HP displayed for my own sake.