Let’s imagine you download the app from the store for your platform and your app crashes at the launch. You open the application again and then, you realize the fundamental feature doesn’t work.
Probably you do what other 60% users do in this situation - remove and forget about an application and start looking for similar ones. Google Play has almost 2.6 mln applications, App Store has about 2 mln applications. Chances that users find other one and stay with it is a more realistic scenario than staying with defected one and trying to open an app for the third time.
Why so many applications are not functional for users? The reason - lack of professional QA team from the very beginning. In the Software Development Life Cycle, every part of the initialization of application should contain a QA Specialist or a team of them, who could catch mistakes of functional assumptions, take care about user experience and create usability documentation from kick-off to maintenance.
It’s never effective. It’s a good practice to do it before the application is transferred to the QA Team but it’s never enough to create a good application. The developer is not objective to his own code and he doesn't see defects as dedicated QA Specialist. The second reason is that developers really don’t like to test their own job. Last but not least the developer doesn’t know a specific technique to test application on a professional level.
Quality Assurance gives you the certainty that you get the product which you ordered. In a time when development is running, there is a lot of changes which testers have to check and report to developers. Continuous verification of application is what testers do every day. If iteration exists without testers there is a big chance that application ends up with a lack of working feature and iteration will be extended.
In practice, testing costs way less than most people realize. If a QA Specialist discovers a bug at the beginning of feature development - cost fixes are cheaper than if it was found after some time. The rule of defect cost (1:10:100) means that bug that is found at the specifications stage is one time the prices after QA session costs 10 times more, but if you find bugs after release - costs are 10-100 times larger. In every sprint, the QA team reports many issues that are fixed at the same time. The fixes, in this case, keep the cost low. In this way, your budget could be the same as you thought at the beginning.
Quality Assurance is an important part of development. The earlier it is involved, the more value in satisfied customers it will bring to your product. Testing on multiple platforms brings you more users which can be using your application.