I have always assumed all the raw meat I buy is full of bacteria that I don't want to eat and can contaminate food which is not going to be cooked.
Another factor is bacteria from the meat which gets on the outside of the raw meat packaging which might contaminate other items in your shopping bag. This is not as bad as it used to be (prepacked raw meat is always in sealed packages now), but it is still present and you should always pack raw meat products in a separate disposable bag. (I reuse it as my kitchen bin liner.)