It's put on there in case an allergen could have by a super slim chance come into contact with the food that ordinarily wouldn't contain it. Very common with nuts because they usually produce more than just peanuts, cashews, ect and tree nuts/peanuts are both common allergies with peanuts being the most common deadly allergy. To avoid potential litigation ultimately though.
I am severely allergic to peanuts but have never avoided a "may contain" product even once. I do avoid products that share equipment with peanuts though especially if they are nuts. I can't have alot of trail mixes or nut mixes due to this, plus the fact they use peanut oil often.
No chik fil a or five guys for me </3