Could be the fabric that the pattern calls for. Also could be in a stress spot that is tight (or going to be tight), so they want to be sure the seam is secure. I find that the first time I make something, I try to follow the directions the way they have been written. After I have finished, I can start making any changes I want. I have patterns that I have changed seam allowances up or down, for my personal sewing comfort. JMHO