The most important reason to properly clean a cutting board is to

The most important reason to washing a cutting board is because it can grow bacteria, if it's not washed with bacteria cleaning soap and hot water.

You did not show us the image of the burger, but i shall try to answer as best i can.  when purchasing food, always consider the ingredients as well as the person who prepared it. consider if they were hygienic people or not, in case the burger was infected with bacteria. no mortal should place an infected burger into their mouth.