<< The software is delivered full of bugs. Why do it? >>
Let's say Microsoft releases Windows XP. The first time you install it, it gives you a BSOD. You curse Microsoft for the bug and vow never to buy any Microsoft products again since they're always full of bugs.
Now think: had a beta-tester reported that bug, Windows XP would be that much better and you would be happier with the operating system.
Beta testers are a sort of buffer between the software and the final user: they experience it beforehand, and report any bugs so that the final user would have a better exprience. In a sense, they sacrifice their time and possibly their machine (if the software fatally messes everything up) just so that the user would be happier.
Without beta testers, buggy software would be shipped directly to end-user, it would take more time (and more unhappy software buyers) to fix the existing bugs, and software progress would be impeded.
Hope that explains.