A once-a-year full count means closing the store or counting through the night, and it tells you that stock figures are wrong long after the reason has gone cold. Between counts, the numbers in your point-of-sale system drift with every unrecorded breakage, mis-scan at the till and receiving mistake.
Cycle counting spreads the work out: count one section at a time, often, and correct the figures as you go. The hard part is the counting itself. Clipboards and handwritten tallies have to be typed in afterwards, and that second step is where many counts stall.
If staff can scan a product and type the shelf quantity in one motion, a section count fits into a slow half hour, and the result arrives as a file you can put next to your system's stock list.