Sketches are great at showing design ideas and working out proportions. They aren’t so great at showing depth and illustrating what design details are straight lines in the pattern.
Sketches – whether hand-drawn illustrations or technical flat sketches – are two dimensional. The final garment, on the other hand, is three dimensional. What appears as a straight line on your sketch, might not be a straight line in the pattern. This is partly due to the body being very much not flat in real life. It can also be because a dart or other contour shaping is hidden within a visually straight seam on the garment.
Waistbands or waist seams, for example, are often drawn as a straight horizontal line in a sketch. However, to get this visually straight waist, the pattern is a curved arc shape. Princess seams and yoke seams often appear straight in the design, but a good pattern often uses these design details to hide contour shaping.
A good patternmaker will know the difference between a line that is visually straight on the body and one that is straight in the pattern. Patternmakers just starting out use more straight lines in the patterns which, ironically, means that the actual garment style lines are less straight on the body. In my experience, there are few actually straight places in a well-fitting pattern.