Google takes some steps towards blocking click fraud, but it's not a priority for them, meaning their fraud prevention measures follow a "one size fits all" approach - something that really limits the effectiveness. After all, not every business is the same or faces the same challenges with invalid and fraudulent traffic!
For example, if a competitor is clicking your adverts hundreds of times from the same PC on the same IP address, then Google will detect this and refund your clicks. However, Google will only provide you with a credit for the amount spent on fraudulent clicks after 60 days - it will not block the fraud permanently and it will still use up your monthly budget!
Plus, Google does not detect the more advanced forms of click fraud. As a specialist fraud & invalid traffic detection platform this is where we come in. 100% of our resources are aimed at stopping and identifying fraud, and our fraud detection process is much more thorough than Googles.
This article explains the difference in more detail.