Let’s all take a moment to talk about gussets – crotch gussets in particular, and what they can and can’t do, because there is just so much misinformation floating around about them. A gusset is a separate piece of fabric sewn into a seam, most commonly seen on underarms (especially in vintage styles) and on the crotch of trousers.
The purpose of a gusset is to increase the range of motion of the limbs nearby – so in the case of the vintage blouse, its because a dolman sleeve doesn’t really allow the arm to raise naturally due to the shape of the bodice/sleeve piece and the limitations on the non-stretch fabric. So a small gusset is added to the underarm will allow the wearer to raise her arms. The same principle applies to crotch gussets – the purpose of a crotch gusset is to increase the range of motion of the legs.