In case you never heard of unqualified traffic, you may want to find out what unqualified traffic is and how it drains your advertising budget.
Often competitors check the ads, promotions and pages of their industry and cause massive amounts of unwanted traffic, which you still have to pay for.
Some users might not behave favourable to Your ads performance. They might not differ organic results from ads and cause unwanted traffic, You have to pay for.
Bots still make up a huge part of the billion dollar clickfraud industry. They are used for generating fraudulent profits for those operating them - at the expense of your ads performance.
Some browser extensions from the "ad block" industry, not only block ads, but click on them to make the advertisers pay. This makes regular, genuine users responsible for unwanted traffic - without them knowing.