For what reason was the sewing machine invented in 1800s?

Like most labor-saving devices, the sewing machine was invented in order to fulfill a need identified by its inventor. In this case, while sewing could certainly be done by hand and had been for many centuries, a sewing machine would fill the need for production to move more quickly and with less effort involved from the seamstresses.
Actually, the sewing machine was not invented in the 1800s. The first sewing machine was invented in England by Thomas Saint in 1790. However, his sewing machine was not produced commercially. The first properly commercial sewing machine was Thimonnier's, which was developed in France in 1829.
An entrepreneur, Thimonnier identified that his chain-stitching machine could be put to factory use to make items on production lines. Specifically, he partnered with a company which made uniforms for the French Army, a pursuit which necessitated quick and continuous stitching. Later inventors were able to make modifications to the original design which allowed for far more extensive and innovative uses of the sewing machine.