A lot of business revolves around policies. For example, eligibility of a user for a product would be a compliance policy. Eligibility for a loan would be a dictated by a credit policy.
Rules can be written as literal code, but then code would be messy as the rule grows. Understanding why exactly a rule failed would be pretty hard and since a strong structure is missing, visualizing would be also pretty hard.
For example, let's take a product eligibility example. Say for example business states that a user is eligible if and only if the user is above 18 years old, and the country code is either IN, india or US.
This rule is pretty simple. But in reality, business rules tend to get pretty large and hairy. It is better to make the policy a rule so that logic & data are clearly separated, and the evolution of the policy is tracked in detail.
For example, the above rule can be written as
(user_age > 18) and ((country_code == 'in') or (country_code == 'india') or (country_code == 'US'))
this would be a neat representation of the policy that a business person would also understand. So, let's make an rule engine where rules like these can be written & parsed.