- To attribute monetary value to non-ecommerce conversions
- To determine the conversion rate
- To calculate ecommerce metrics
- To determine the number of visits to each of your page
Explanation: Assigning a Goal value is optional but Google recommends you do so to help monetize and evaluate your conversions. Each time the Goal is completed by a user, this amount is recorded and then added together and seen in your reports as the Goal Value.
You can read more here: https://support.google.com/analytics/answer/1012040?hl=en