There are substantially more details included in Matthew’s Parable of the Weeds, recorded in verses 24-30 of chapter 13, than in Mark’s Parable of the Seed Growing. Both begin by comparing the Kingdom of God/Heaven to a man sowing seed/scattering seed on the ground of a field. Matthew goes another direction with his parable, though, and makes the distinction that the seed is good. Mark makes no distinction between the quality of the seed because, it seems, the point of the parable is not to address the quality or lack thereof.
The point of Mark’s parable seems to be, in contrast to expectations that the last judgment (sickle and harvest) would come soon, instead that the commencement of the Kingdom of God would inconspicuous and its growth would be slow (first the blade, then the ear, then the full grain in the ear) but come to fruition in due season (when the grain is ripe, at once he puts in the sickle, because the harvest has come), and then the Kingdom of God will come. Matthew’s parable, on the other hand, draws a distinction between good and bad seed. With the bad seed (sons of the evil one) come weeds, sown among the good seed of wheat (sons of the Kingdom). The bad seed are sown by the enemy (the devil) of the man (Son of Man). At early growth, the weeds and the wheat are indistinguishable, but as they reach maturity you can tell them apart. Attempting to gather the weeds would harm the wheat, so they are allowed to grow together (in the world) until harvest (the end of the age, Day of Judgment). The prolonged coming of the Kingdom of God isn’t the point here, but the fact that there are good and bad seed, and the explanation for why the bad seed are allowed to continue. This could be seen as an expansion on Mark’s parable, but the clearest reason for Matthew’s parable being different is that the point the parable is making is different. The two parables are both talking about the Kingdom of Heaven, but they’re making completely different points.
Matthew’s account of Jesus explaining the Parable of the Weeds has a much more explicitly elevated view of Jesus than Mark’s parable. Jesus is the Son of Man, and he is the one sowing sons of the Kingdom and the one inaugurating the Kingdom of God. Clearly this is a portrayal of the scene in Daniel 7, after the Ancient of Days gives everlasting dominion over the Earth, glory, and a Kingdom to one like a Son of Man. Matthew, in his account, is clearly identifying Jesus with this Son of Man figure.
Matthew’s Parable of the Treasure continues the theme of talking about the Kingdom of Heaven. Here another point is made as to the inconspicuous or hidden nature of the Kingdom of Heaven. However, the main thrust of this parable seems to be more about the surpassing worth, as treasure, of the Kingdom of God compared to any sacrifice one could make to get it. Matthew’s Parable of the Pearl of Great Value is also talking about the Kingdom of Heaven, but unlike the man who stumbled on hidden treasure, this man was is a merchant is earnestly searching for fine pearls. But when he found the pearl of great value (the Kingdom of Heaven), he has the same reaction as the man in the field and sacrificed all he had to get it. Matthew’s Parable of the Net is essentially making the same point as his parable of the weeds. Making the same point that good and bad fish won’t be sorted until the final harvest, and evil won’t be totally removed from the world until the end of the age. Making these points about the Kingdom of Heaven would do well to explain Jesus’ rejection by the Jews and his movement’s seeming defeat at his death.