The Waistcoat

I’m currently at the step of sewing darts into the waist coat. In the video Rory sews a small piece of fabric into the dart tip. Is the purpose of that fabric just so you have something larger to press the seam allowance flat?

Thanks!

I just started the waistcoat course. Thankfully my chest is a 42 so I could use Rory’s measurement from the video, but using the measuring chart I don’t see how the front shoulder width is determined. I have the pattern drafting book but I haven’t taken that course yet. Is there a way to determine it or measure it? Thanks!