Other than the fact that Santiago's dream involves the simple things in life, the gypsy woman also realizes that his dream is "in the language of the world." She goes on to say that she "can interpret it, but the interpretation is very difficult" (14). In reality, however, the gypsy is not experienced enough as far as omens and other mysteries of the world are concerned to give him a valid opinion. She knows the dream is important, but she can't say exactly why.
The Alchemist, on the other hand, has the wisdom, education, and experience necessary to teach Santiago about the language of the world, omens, and other bits of wisdom. For the most part, the dream is just a tool to open the boy's mind to possibilities outside of his realm of safety. Without the dream inciting him to search for answers, he may not have been ready to receive Melchizedek's invitation to do whatever it takes to achieve his Personal Legend.
The gypsy, though, only knows that the dream is important because it is part of the language of the world and that is why she makes the boy swear that he will give her 10% of his treasure if he finds it. Fortunately for the boy, his journey leads him to the Alchemist, who can teach him how to interpret his dream for himself.