Bills that arrive once a year can make one month look unusually expensive. Spreading them across your budget lets you set money aside before the bill arrives.
An example with numbers
A $720 yearly insurance premium works out to $60 a month. If you start setting aside $60 in January and the bill is due in December, you will have the $720. If it is due in three months and you start from zero, you would need $240 a month until then.
Try it with your numbers
Add the monthly set-aside to your expense categories and check what you have left. When you pay the bill from that reserve, avoid counting the same expense twice.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.