Functional programming in Go can enhance code readability, simplify testing, and reduce complexity by eliminating states and mutable data, adhering to principles such as using pure functions, avoiding side effects, and maintaining immutable data. Pure functions consistently produce the same output for the same input, which aligns with the concept of idempotence, and function composition allows for the creation of advanced functionalities from simple, pure functions. Functional programming discourages shared states to prevent side effects, instead favoring immutable data structures to derive new states. Despite Go not being inherently designed for functional programming, the approach introduces a paradigm shift that offers a bottom-up programming style focused on modularization and pure functions, improving code quality but sometimes compromising performance. Examples in Go include updating strings, arrays, and maps functionally, using higher-order functions and currying, and employing recursion to avoid loops, illustrating the trade-off between better code quality and performance.