By definition, "". But Software Testing’s contribution is broader than its original definition. An experienced tester not only finds defects but also adds suggestions to bring out quality in the system.
In one of my previous projects, I suggested that the architect added an aggregated table to improve the performance of a report. When it was implemented, the performance of the report improved greatly. I personally feel Quality Engineer is a better term for a tester as he brings in a lot of quality into the system by identifying areas of failure and improvement.
There has been a greater awareness in organizations on testing in recent years. Organizations understand the cost of not testing and implementing for every product they are building.
In brief, testing is essential as:
1. It identifies defects and hence, gives scope for greater quality to be built into the application
2. It ensures the functional as well as the nonfunctional requirements have been properly met in the application
3. It ensures that the product is giving business value to the customer
4. It ensures that the possible product risks and known issues have been addressed