The line is the reason you don’t lose a day now. When you circumnavigate the world westward, the earth has rotated one less time for you than for everyone else, so it appears you have lost a day. By artificially switching the day at the date line you avoid this kind of slippage.
On a ship, you also keep the same time zone as local time for the area you are steaming through. Going consistently westward to get somewhere is a bonus, because every other day you get to sleep in an extra hour.