Why are rule inputs used in an expression?

Rule inputs are essential for passing values into an expression at runtime, allowing for dynamic functionality based on user input or other variable factors. This capability enables developers to create more flexible and responsive applications, as the outputs of the expression can change according to the inputs provided when the rule is invoked.

Using rule inputs means that the expression can operate on a wide variety of data without being hardcoded, resulting in reusable and modular components. This is particularly useful in scenarios where you might need to perform similar operations on different datasets or based on user selections, ultimately enhancing the overall user experience and reducing redundancy in expression design.

In contrast, the other options do not align with the core purpose of rule inputs in expressions. While literal values, database queries, and functions may be utilized in various contexts within Appian, they do not fulfill the specific role of enabling runtime input into expressions, which is paramount for dynamic application behavior.