In a time where human mostly ate most uncooked vegetarian food, unsweetened and unrefined, dental hygiene took mostly care of itself. There was no sugar for the bacteria to live from and the raw food cleaned our teeth. Today almost all foods are refined, include sugar or starch that is converted to sugar in our mouth. While eating better does help we now need to support our teeth by cleaning them and reducing the amount of agresive bacteria in our mouth.
Since a dentist can only patch a cavity, but neither our dentist nor our body can ever repair one, we should avoid cavities. The first 4 years of a tooth are especially sensitive, if they survive this period cavity free we can hopefully keep them cavity free all our life. For this we keep a good oral hygiene, not only to keep the teeth clean but also to preserve an oral flora that is mostly free from bacteria that can lead to caries. So we should start as early as possible, first by not giving our bacteria to our kids then by keeping their flora clean. Over the coming years this will prevent many sufferings from pain, support our immune system and save a lot of money.
The mouth is a huge area of sensitive skin tissue, thus the risk of an infection is high and inflamed gums are quite common over a lifetime. Infections in the mouth weaken the immune system and the mouth can be the gateway for many bacteria that then go on infecting other body parts through the blood circulation.
Of course the perfect situation is if the parents already are careful with their oral hygiene and keep a good oral hygiene for the baby early on. The kid then learns to brush and keeps his teeth clean to never get a cavity. But no matter what your actual situation it is never to late to preserve what is left and improving the oral flora. Thus keeping the mouth free of dangerous bacteria and infections has an effect on the whole immune system and various health risks. And finally things like reducing the amount of sugar we eat is of course something good for the whole system not only the teeth.