It could be the advert is actually part of the video, so anyone watching the video will also see the "advert".
It is an easy way for people to get "free" advertising.
Create a funny video of a cat falling over, put your advert in the corner of the video, then forward it to all your friends and, like lemmings, they will forward it to all THEIR friends and so on so it goes viral.
Free advertising !
btw NEVER trust any advert like this that says your drivers are out of date (or you have a virus, or your PC is full of porn, or all sorts of other "scare" messages. They are all scams).