The activity of germicides against microorganisms depends on a number of factors, some of which are intrinsic qualities of the organism, others of which are the chemical and external physical environment. Awareness of these factors should lead to better use of disinfection and sterilization processes.
The key reason sterilization is important is the way it prevents the potential spread of harmful bacteria and other pathogens which might cause disease. Conditions like HPV, hepatitis and certain others are known to commonly spread through the sorts of tools and surfaces which are used in dental and other medical treatments (also in some drugs which use needles).
In addition, proper sterilization stops the spread of any individual illnesses a prior patient may have.