In Rust, the `unsafe` keyword is used to indicate that a block of code or a function contains operations that are not subject to the usual safety guarantees provided by the Rust language. It allows you to bypass some of Rust's safety mechanisms and perform low-level operations that require manual management of memory, concurrency, or other system resources.

The purpose of the `unsafe` keyword is to provide a controlled mechanism for writing unsafe code within an otherwise safe Rust program. It enables you to write code that interacts with the underlying system or performs operations that cannot be expressed safely in the Rust type system. Some common use cases for using `unsafe` code include:

1. Dereferencing Raw Pointers : The `unsafe` keyword is used when dereferencing raw pointers obtained from outside the safe Rust code. It allows you to perform low-level memory operations and access data directly without the safety checks enforced by the Rust borrow checker.

2. Calling Unsafe Functions : Some functions in Rust are marked as `unsafe` in their API, indicating that they have certain requirements or behavior that cannot be verified by the compiler. When calling such functions, you need to use the `unsafe` keyword to indicate that you are aware of the risks and have taken the necessary precautions.

3. Implementing Unsafe Traits : Traits in Rust can have associated functions or methods marked as `unsafe`. When implementing such traits, you need to mark the corresponding functions or methods with `unsafe` and ensure that you meet the requirements specified by the trait.

4. Interfacing with Foreign Code : When working with foreign code written in other languages like C or C++, you often need to use `unsafe` code to call into and interact with the foreign functions and data. This allows you to handle the differences in memory layout, calling conventions, and other low-level details.

It's important to note that using `unsafe` code introduces the potential for undefined behavior, such as memory unsafety, data races, or other issues that could compromise program correctness or safety. The responsibility lies with the programmer to ensure that the unsafe code is used correctly and does not violate the Rust language's safety guarantees.