An appraisal is necessary as lenders that will fund the deal hire appraisers to obtain a professional opinon of value. This is done to protect the lender and ensure they are not lending more money than the property is worth.
This is why it is extremely critical that when submitting offers on property, you do not over pay, assuming your planning on securing financing that is. For example if you submit an offer for a million dollars and it gets accepted but the appraisal comes back for $850,000, you will be liable for the balance of $150,000 on closing.
So long as you are in tune with market values you should not have an issue with a low appraisal however it’s important to be aware of this and avoid over-offering.