Baby booties help

Ok wanting to make baby booties. Found some really cute ones on the lion website. I understand the instructions all the way to row 4 and 5. How/why do you cast on in the middle of a project??:knitting:

RIGHT BOOTIE
Beg at center of sole, cast on 28 (32, 36) sts.
Row 1: Purl.
Row 2: Knit.
Row 3: P12 (14, 16), (m1, p1) 4 times, m1, p12 (14, 16) – 33 (37, 41) sts.
Row 4: Cast on 3 sts, knit to end of row.
Row 5: Cast on 3 sts, purl to end of row – 39 (43, 47) sts. Knit 8 (10, 10) rows.

Shape Top
Row 1: K13 (15, 17), ssk, k9, k2tog, k13 (15, 17) – 37 (41, 45) sts.
Row 2: K13 (15, 17), ssk, k7, k2tog, k13 (15, 17) – 35 (39, 43) sts.
Row 3: Knit.
Row 4: K13 (15, 17), ssk, k1, sk2p, k1, k2tog, k13 (15, 17) – 31 (35, 39) sts.
Row 5: Knit.
Row 6: K7 (8, 9) and sl to a holder (for button tab), bind off 7 (8, 9) sts, k3 and sl to a holder (for t-strap), bind off 7 (8, 9) sts, k7 (8, 9).
Strap
Continuing on last 7 (8, 9) sts.
Row 1: K7 (8, 9), cast on 16 (19, 21).
Row 2: Knit.
Row 3 (buttonholes): Knit to last 6 (9, 9) sts, (k1, yo, k2tog) 2 (3, 3) times.
Bind off.
Button Tab
Sl 7 (8, 9) sts from holder back to needle, join yarn and k 3 rows. Bind off.
T-Strap
Sl 3 sts from holder back to needle, join yarn and k 18 (22, 26) rows. Bind off.

Maybe this pattern is really beyond me! When it jumps to shape top do I just start another piece? Do I pick up stitches?

You would use the knit or cable CO; move the right needle with the working yarn to your left hand, cast on the sts, then leave it there to knit the next row. It’s a way to add additional sts to extend and make the row longer. You can’t just increase that many in one spot, they’ll bunch up.