Introducing new features, updates, or enhancements can inadvertently introduce functional bugs. Testing ensures these additions integrate seamlessly.

Incompatibility issues between different modules or components can lead to incorrect or unexpected behavior, which functional testing can address.

Changes made to fix one issue might inadvertently impact other areas of the product, which functional testing can catch through regression testing.

Functional issues might lead to performance bottlenecks or slowdowns, indicating a need for testing to identify and rectify such flaws.

Testing in a static environment doesn't always reflect real life. Many issues can arise from changing networks, locations, and so on. To ensure your app works as intended, testing its behavior under different conditions is crucial.

Functional testing becomes crucial when the product needs to function consistently across different web browsers or platforms.