Yup. Software developers and software testers -- at least if they're both good -- use different skill sets. Technically, testing things before you ship them is a cost center, but most companies do not want to spend the time and money to make practically-defect-free software before testing: It requires that someone who knows what they're doing set up a strong development process, that developers consistently think hard about what they're doing, and that a lot of risks are eliminated or realized before things go out the door; in practice, this means a fairly conservative and slow development process. So if you're not spending money on that kind of process, it is very much worth it to spend money on good testers.
Unfortunately, companies seldom pay software testers anywhere near the value that a good tester brings to the company. In part, that's because it is very hard to tell who will be a good tester before they start, and bad testers cost more than their direct salaries, because they will send others down rabbit holes. If someone is willing to work as a tester for $X, where's the incentive to pay them $X*2 once they have shown that they are actually worth that much or more?