What is the purpose of a service fee?

Excuse me, could you kindly elaborate on the essence of a service fee? What precisely is its objective, and how does it contribute to the overall financial transaction process? Is it a mandatory cost that's inherent to the service being provided, or is it negotiable? Furthermore, does the amount of the service fee vary depending on the specific service being offered, and if so, what factors typically influence this variation? I'm genuinely curious to understand the rationale behind this charge and its significance in the world of finance and cryptocurrency.