Shaping back neck and shoulders

My pattern states as follows: I don’t understand how to do this. I have worked to ‘Dec 1 st at neck edge of next 3 rows, ending with RS facing for next row.’ I don’t see why I need to break the yarn when the pattern then says rejoin it. Can someone please explain what I need to do. I now have 15 stitches for the right shoulder and all the remaining stitches on the needle as well.

Shape back neck and shoulders
Next row (RS): Knit until there are
17 [18: 20: 21] sts on right needle K2tog, turn and work on these 18 [19: 21: 22] sts for right shoulder.
Dec 1 st at neck edge of next 3 rows, ending with RS facing for next row.
Break yarn and leave rem 15 [16: 18: 19] sts on a holder.
With RS facing, rejoin yarn to sts from holder, cast off 15 [17: 17: 19] sts, knit to end.
19 [20: 22: 23] sts.
Dec 1 st at neck edge of next 4 rows, ending with WS facing for next row.
Break yarn and leave rem 15 [16: 18: 19] sts on a holder for left shoulder.

Welcome to KH!
The pattern isn’t very explicit here. There are 2 stitch holders implied, one for the unworked sts in the first row and the second for the 15sts of the right shoulder. (It’s perfectly fine not to put the unworked sts from the first row onto a holder but to leave them on the needle while you work on the right shoulder.)

Now you’re being asked to rejoin yarn to those unworked sts from the first row. The bind off is of the center 15sts so that you can knit across the left shoulder and begin its shaping. When you rejoin the yarn strand, leave about a 6" tail to weave in later.

Thank you SO much! It’s all much clearer now.