GraphQL variables allow developers to build queries and mutations that accept values from their application or users, improving type safety by restricting what kinds of values can be passed to them. Variables are defined separately from the query itself and can be used in both queries and mutations, with default values provided for some variables. The use of GraphQL variables enables predictable code, reducing problems such as unexpected input and injection attacks, while also providing a self-documenting nature that makes it easy to see what's going on and make adjustments to get the exact data required. By using typed languages like TypeScript, developers can ensure consistency across their front- and backend, making it easier to develop complex applications and improving app reliability for end users. Additionally, GraphQL variables can be used in combination with schema stitching to combine different APIs into a single endpoint, streamlining application code and improving responsiveness by reducing backend requests required to keep data up-to-date.