As far as I know they were used for everyday sewing. The chain stitch if finished off properly is just as strong as a lock stitch. You just have to be aware of that end!
Now adays most folks use them for either mock sewing to see if the pattern worked and then snip the thread and unravel the hole thing without seam rippers to make adjustments or sew it again with a lock stitch, OR they use it for decorative free motion embroidery! They sew upside down so the pretty chain stitch is on top!
Can you tell I've looked this up? Hahaha!