Хорошие статьи на этот счет
https://itnext.io/front-end-testing-strategy-5fddfd463feb
https://kentcdodds.com/blog/common-testing-mistakes#mistake-number-2-100-codecoverage
https://andersenlab.com/blueprint/win-the-testing-trophy
Примеры когда нужна
The sooner we get feedback from real usage, the less time/money/opportunity we spend on behavior that doesn’t matter.»