Good grooming and cleanliness helps you stay healthy. Cleaner you are,the healthier you'll be from less germs. So, good grooming habits go hand in hand with good health.
There are a lot of different areas where it is beneficial. Being well groomed and healthy makes you more likely to get and maintain a job, friends, and relationship. Beyond that, if you are healthy you will feel better and if you are well groomed it can do the same as it typically makes you feel more confident. You just seem to be a person who is more approachable and even more trust worthy in cases when you are well groomed and healthy.