Software Testing is necessary because we all make mistakes. Some of those mistakes are unimportant, but some of them are expensive or dangerous. Ideally, we should get someone else to check our work because another person is more likely to spot the flaws.
It’s required to stay in the business.
Users are not inclined to use software that has bugs. They may not adopt a software if they are not happy with the stability of the application.
In case of a product organisation or startup which has only one product, poor quality of software may result in lack of adoption of the product and this may result in losses which the business may not recover from.
If the customer does not find the testing organisation reliable or is not satisfied with the quality of the deliverable, then they may switch to a competitor organisation.
Sometimes contracts may also include monetary penalties with respect to the timeline and quality of the product. In such cases, if proper software testing may also prevent monetary losses.