Requirements depend on the platform. Each of those platforms you named may have their own requirements, so that's where to check on that. Offhand, though, I don't recall seeing ESRB ratings on the App Store, but I have observed that I am not particularly observant. ?

As for benefit. Different audience segments draw conclusions (positive/negative) about ratings. Parents and grandparents may decide not to buy a game for young child if the rating is too high. Hardcore gamers might be turned off by a family-friendly rating. Those are just two segments, all I can think of offhand, where a rating might matter as to buying decision. You need to know which segments matter to your business, and satisfy that constituency.