1. Testing enables you to certify your design. To certify means that you are validating your design in objective terms of quality, safety, and functionality. You might have seen the different groups out there that deal with design quality like ISO, DIN, IEC, NEC, ANSI, CAS, ASME, and other "alphabet soup" agencies.
2. Testing allows you to proactively screen your design against trolls, customer returns, and bad reviews. The more you test, the less you are leaving to chance. The more you mitigate your design risks the less of chance you'll encounter issues.
3. Testing is not the same as assembly. Assembly is the physical assembly of the prototype or final product for future testing. Please don't get sucked into the school of thought that assembly is a substitute for real world testing. Testing is different and it should be viewed as such.
4. Testing doesn't always have to take the form of labs, wind tunnels, and other traditional testing areas. Focus groups can provide reviews, complaint-worthy issues about your design, and give you some constructive feedback. In the end it's better to have a focus group negatively review your design than have a real customer do it.