If you only sew a couple of millimeters off the desired seam line, it might not matter on a big project. However, on a small doll arm, foot, neck, or round doll head, it could change the shape of the whole part. A seam allowance is not just a strip of extra fabric around a pattern piece. It is there for you to sew, trim, turn, and shape the part so the shape remains close to the original pattern outline.
You could do a quick exercise with a small doll pattern piece. Trace it on some scrap fabric twice, then clearly mark the seam line. Sew one of the samples directly on the seam line, but sew the second sample slightly to the inside or outside of the seam line. Now turn both pieces right side out and put them together. The slightly shifted seam might cause one of the pieces to be slightly smaller or skinnier or sharper. You might find it easier to see the difference in the fabric than to be told the seam allowance needs to be uniform.
Round shapes on a doll pattern, like a doll’s head, hand, or foot, need the uniformity of the seam allowance. They are made up of many short, different directional changes along the seam line. If the stitching goes inwards at one point, the curve might end up with a flat area. If it goes outwards, the part might be fuller than expected. Smaller, more controlled stitches work better around these tighter curves, because it will allow you to stitch along the seam line without having to push the fabric through a sharp change of direction.
The fabric outside the seam also makes a difference in how well the part will turn. When you sew the seam, there will be seam allowance inside the piece, which can bunch up in a small curved area and force the part to end up bigger than it should. Depending on the type of fabric and the type of pattern, you may be able to trim back the bulk of the seam allowance and clip or notch into the curved areas so that when you turn it, the seam opens out more evenly. Just remember not to cut into the seam line itself, otherwise the seam will fall apart. With narrow doll pattern pieces, you may want to trim back the seam allowance more carefully, rather than cutting all at once.
When a beginner is focusing very closely on the edge of the fabric, they sometimes forget to look at where the seam line actually is. This can become even more of an issue when you are sewing two pieces that will be sewn together and turned right side out, like two doll legs or two doll arms. One leg may follow the seam line carefully, while another one may be using the outer edge of the fabric as a guide and end up being a different size or shape. Marking the seam line before you sew will make it much easier to follow accurately, particularly when the edges have complicated curves or a small turning opening.
It helps to check the seam allowance before you stuff the doll. When you have both pieces of a pair sewn and turned, lay them down side by side and see if they match up. Are they the same size? Do they look the same? Does one have a bigger curve than the other? Can you feel along the seam and see any places that dip inwards or pop outwards? If the differences are coming from the seam, you will probably find it easier to fix now than to try to reshape the doll part later with fiberfill.
As you get better at sewing and get more confident with the machine, your goal is not to sew every single stitch with perfect, robotic consistency. Your goal is that when you turn the body of the doll right side out, it has the right overall shape and proportion, without any weird dents, uneven curves, or pairs that don’t match. Next time you sew a doll pattern piece, keep an eye on the seam line when you are going through the tightest curve, and see how just a few small movements of the fabric can affect the finished shape.