For most accounts, rank is usually a pretty good indicator of their mmr (to within a tier or so anyway, which is within queuing restrictions). Queue restrictions are there to make sure roughly equal mmr skill people are playing together, so in normal circumstances allowing a silver to queue with a plat would mean a large mmr gap between you, which is not desirable.
Those on exceptional win/loss streaks might find themselves with an MMR beyond 1 tier of their rank, and hence a silver ranked player might find themselves in a plat game, but they're in a special circumstance where they've proven to the system that they can play at a plat level, so it's not a problem. (Sometimes the other direction happens too, where it's not a silver in a plat game, but rather a particularly poor playing plat player in a silver game, or a mix of those.)
In an ideal world ranked queue restrictions would be purely mmr based so the low ranked smurfs could queue with their high ranked pals of a similar skill (and not play with their similarly low ranked but unskilled pals they may be attempting to boost), but people could manipulate that to figure out what peoples MMRs actually were, and riot would prefer to keep that secret.
