Back then, it really had nothing to do with quality. There were probably loads of games on the NES and SNES that sucked, perhaps as much as the minigame collections all too common on the Wii. Rather, it was partially about censorship, as Nintendo knew all about Custer's Revenge and the rest of the unlicensed porn games on 2600 and the Big N wasn't going to let those kinds of games ruin the NES' family image, and it was partially about making sure Nintendo could control the flow of games, preventing a 1982-like overload of games where no store could have them all (one cause of the Crash). These were part of Nintendo's overarching purpose: make money, and let's not shoot ourselves in the foot by allowing another crash. Of course, as games became more expensive and complex to develop and as gamers grew old enough that nobody thought they needed censorship, the quality seal became less important to the point where it's now almost meaningless, an artifact from the old days.