Those letters are helpful in making the snap decision as to whether to even bother investigating the content further. I can't give every game/book/movie/whatever proper review, so it helps to know that some sane adult marked it with an easily-understood cue. If there's an "M" or "T" on the box, I don't want to waste another second wondering if it's suitable for my toddlers.
Example: "Happy Tree Friends" is advertised & starts with cute forest animals in simple & colorful renderings; an appropriate parental cue helps prevent having to explain other definitions of "rendering" to a kid who can barely talk and is wondering why seconds later that cute squirrel is shrieking, spewing red, and has a bad case of the ouchies. A prominent "T" would be helpful to those adults who don't know the show and are being pestered by a 3-year-old to watch it on Netflix.