Software developers are usually pretty innovative designers. This is because they understand the system well and work to improve its design, as they build the software according to requirements. Quality Assurance (QA) is more focused on finding the imperfections, identifying problems, and ensuring business requirements are met. Developers write the code for changes in design and updates in software, but they tend to miss out on understanding exactly how their changes can alter other aspects of the overall product or software.
Usually developers are more focused on finishing a certain desired aspect of the code. The piece of code they work on could have an impact anywhere throughout the process, which is why we have the concept of end-to-end testing. This is where it becomes truly vital to have QA engineers on board. Apart from executing the usual test cases, QA engineers have insights into system architecture and know the overall functional flow of the system. In end-to-end testing, applications are tested for critical functionalities, such as their communication with other interfaces, database, the overall network, and other external applications.
Software developers can shorten the process of coding by taking innovative shortcuts. Even though software developers test their code, they test to make sure their code is functioning just fine. The difference with QA engineers is that they test the code with strategic and specific failure cases. They have an in-depth view of the places where there could be few bottlenecks, and they make use of their experience by targeting specific instances that are bound to fail, and can be overlooked at the time of coding.
Imagine a scenario with smartphones. We are all extensive users of the smartphone, whether Android or iOS, and we get system updates every once in a while. Now, while the developer would build in the extra features that come with the new update, the QA engineer would already be several steps ahead. The QA engineer would test to verify that the update is in line with the phone model, that the user can download the update and install it with ease. In addition, it is important that the interaction of various applications should be seamless with the new system update. Developers tend to skip quite a few paths and that is because they consider it redundant when they know that the code already seems to be working just fine. If end-to-end testing is not done, imagine the amount of backlash an organization would receive by users, due to system updates being endlessly faulty or delayed!
QA engineers are naturally inclined to sense a misfit in a big picture. They catch software bugs faster and in earlier stages, because they have been trained to do so their whole career. QA engineers also do not take end-user testing lightly. They know well what to look for from the system as a user who does not typically know the internal architecture of the software. Developers have way too much knowledge and this hinders their view to look at the system with a clean set of eyes. As a result, they unknowingly apply short-cuts or take detours to perform a task that the end-user would never even think of. This could cost a lot of time and effort for the user!
In a nutshell, developers can perform certain segments of testing to perfection, such as unit testing and effectively constructing edge-case test scenarios. A QA engineer can speed up the process much more effectively by testing thoroughly the entire system, including each path of response. QA engineers are equipped with domain knowledge, and will take the time to go greatly into detail. With the end-user in mind, they will always be several steps ahead in effectively catching software glitches and saving the company millions of dollars and hours of precious time.