Few reasons really but in terms of which workplace? I work in a pharmacy and I would say in relation to that, personal presentation covers wearing correct footwear - which ensures that you don't trip up causing personal injury to yourself, or in the case of wearing incorrect footwear like sandals etc not having the protection against any items you may drop on your feet, or sharp edges etc.
Also washing hands frequently to prevent contamination of medicines etc with germs and the like. Same reason should tie hair back really.
Not wearing jewellery etc which could have germs on etc which could pass on.
Wearing protective clothing like overalls etc not only to protect your own clothing but also to stop outside microorganisms being brought into the workplace, particularly when working with medicines like I do myself. Protect clothing in terms of loose articles getting caught in drawers, units etc.
Just a few examples, hope this helps KD
Good hygiene is required to prevent cross infection and disease spreading.
Personal presentation is important because they serve as a protection or prevention from injuries.
Keep hands and nails immaculately clean.
Clean hair and body.
Don't eat things that can cause offense like garlic, onions.
If you smoke you can smell very bad especially to a non smoker - or a reformed one.
If you smell bad then clients will not return and it is most unpleasant for your colleagues.
So that you do not get sick or infect others with harmful bacteria or germs that can cause illness. Wash your hands, keep your hands away from your eyes, nose and mouth, and use paper towels on bathroom door handles. Don't come to work if you have a cold or flu. Sneeze into a handkerchief or your elbow. Cover your mouth when you cough.