An AdWords account that starts off making a worthwhile profit for the owner is often neglected and, within a year or two, is losing money. Potentially a lot of money. I have seen it happen again and again. If you are running a Google AdWords campaign, you have to at least monitor it. Better still, actively maintain it. Otherwise the rot will soon set in.
Here is an example of an AdWords campaign that was professionally set up and then left to coast, unmanaged. You can see that the cost per converted click (blue) rose, while the number of conversions (orange) fell a little. The average cost per conversion (trial) rose by a factor of 3 in just 4 years. What was once a profitable account was now wasting a lot of money. Ouch.
The main reason for the rot is that Google are continually changing AdWords, and they may opt you in to new features without asking you.