What is an independent laboratory and why would its results be considered more reliable than the results from a laboratory paid by a company selling a particular product?

Let's imagine a study that investigates the effect of tobacco on health. If such as study was conducted by a laboratory paid by the tobacco industry, you would be right to doubt in the results! That's why we need independent laboratories: laboratories not sponsored by people who would benefit or be harmed by the result of the work in this laboratory.