I'm not sure why. There are a few reasons I could think of. One is that p1 k1 may work out better after pieces are seamed together. Two maybe the stitches are aligned well with the pattern e.g. cable pattern. Three, just a personal preference.
Thank you for your question. have a wonderful day!
Ratcha
Shape Shoulders. Cast off 89:11:12:14 sts at the beg of the next 64:6:2:6 rows. 5173:55:109:59 sts. Cast off --10:--:13:--sts at the beg of the next --2:--:4:--rows.