Understanding seam allowance is an important part of learning to sew. If you don’t sew with the correct seam allowance, your finished item may not look good.
There is no single standard seam allowance, and can even vary for different seams in the same garment. Every pattern designer has their own preferences for seam allowance.
A larger seam allowance means there is more fabric on both sides of the needle for the feed dogs to “grab”. This can make the fabric feed more evenly and easily.
A smaller seam allowance also creates less bulk. This can be particularly useful on tight curves and small pieces. Note: The bulk can be reduced by trimming the seam allowance after sewing.