Apple trees aren't inbred - the seeds will not grow into the same variety as the parent. Usually, store bought apples will have seeds that produce inferior fruit since crabapples are used as a pollinator (apples aren't really all that self fertile) - so the seeds are hybrid crabapple seeds. You'll still get a tree that produces fruit that makes a good jam, but not great eating apples. Its not true that you're guaranteed to get an inferior quality fruit from planting a seed - otherwise we wouldn't ever develop new varieties - but the conditions that made your supermarket apple happen will result in a eating apple/crabapple hybrid more often than not. Because they don't really self pollinate, you're not going to be able to create a breed of apple that produces offspring generally identical to the parent plant, so the only way to keep a variety going is to clone it with grafting techniques.