In American English, a vest is a sweater with no sleeves and buttons down or pulls over a shirt; in British English that's a waistcoat.
None of this matters to me, I love vests because they're warm but not bulky under my raincoat and there's no arms to bunch under that raincoat. But the best part is not getting stranded on sleeve island.
Vests also give you more flexibility in movement and they don't take up as much room when packing. I find there is less wear and tear, no elbows or cuffs to wear out.
Do need more reasons, theres less yarn to buy and less time spent knitting. I'm on to my next project in record time. A medium sized woman's pullover in worsted weight typically takes about 1400 yards, a vest only takes about 900 yds. Construction detail will affect the actual yardage needed.
Most of all have fun and enjoy the process. Andrea
Favorite Vest Patterns
Weekend Slipover V-neck by PetiteKnit, bulky weight
Vest No. 1 by My Favourite Things, worsted weight
Sleeveless by Lone Kjeldsen, sock weight