If you do not have a zero balance by 31 March, the amount you can salary package in the following FBT year may be reduced, as any money left on the card will be carried over to the following FBT year. This could affect your regular payments made from your salary packaging, such as your home loan or rental payments (if you have them paid from your salary packaging funds).
For example, if you have a CBB Meal Entertainment and Holiday Accommodation Card with a balance of $650 at the end of the 2020 FBT year, the amount you can package onto your card in the following 2021 FBT year is $2,000, rather than the maximum benefit amount of $2,650.
It is more beneficial for you to have a zero balance on your salary packaging account/cards by 31 March each year, as this will enable you to package the maximum benefit available to you in the following FBT year. This means more savings, and more money in your pocket.