Curved needles work very well when sewing a coptic stitch. It allows the stitching to be tight. The curve allows you to "hook" around as you are sewing. You can also make your own curved needles out of straight ones. I like to use darning needles in size 1 to make my curved need
Book Arts Community
Sewing Repair: Using A Curved Needle
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bookbinding by dirk seynhaeve, via Behance
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AAA L2 CURVED FLAP WITH UNDERSTITCHING
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12: Curved Needle