The main idea about growing crystals is that as the amount of solvent decreases due to evaporation, the excess solute has to drop off the solution. When that happens it has to find a place to park itself. When the very first atoms precipitate they are going to find an impurity to stick to. Remember, no matter how pure the solution is, they will find something to attach to. So, the first step is to grow the seed crystals. They have to be as perfect as possible, because, if they are flawed in any way, the crystal will be flawed as it grows and these defects will get amplified. Another thing to remember is that the slower the crystals grow, the more time they have to arrange the precipitated atoms and hence the more perfect they will be.