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Unit tests are written to **guarantee the reliability and predictability of the code throughout the whole life of the project**. They give the developer confidence that individual functions and modules work correctly and do not break when changes are made.

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### **Main reasons why unit tests are needed**

1. **Early bug detection**
Unit tests catch bugs at the function level while the system is still small. This is far cheaper and faster than looking for the error later in a large module or, worse, in production.
2. **Confident refactoring**
When code is changed or improved, there is a risk of breaking something. Tests act as "insurance": if something goes wrong, the tests fail and you see the problem immediately.
3. **Long-term project stability**
The project grows, developers change, the logic gets more complex. Unit tests capture the expected behavior of functions, so the system stays stable even years later.
4. **Documentation of behavior**
The tests show how a function is supposed to behave in specific situations. They work like living documentation that does not go stale (unlike text descriptions).
5. **Isolation and predictability of logic**
Tests force you to write modules that are independent, clean and free of hidden side effects. This improves the architecture.

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### **Summary**

Unit tests are a tool that helps **make code safer, more stable and longer-lived**, reducing the number of bugs and the stress of development.

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