When you use an item from your hotbar (say drink from a canteen or eat from a can), the character puts away the item after one use. Because of this you can't take another swig from the canteen or eat another bite from your can without having to bring out the item again.
The automatic hiding of the item also messes up animations when you go from eating a can to bringing out your melee weapon/gun.
The eating animation finishes, you switch to the gun, the gun appears, the character puts away the gun, the hotbar goes back to whatever item you had before so you have to select your gun again, taking it off your shoulder a second time. It also makes the hotbar feel less responsive, since you often have to select items twice or more because of all the extra animations playing.