Analogy for banked sick leave

Suppose your employer gives you 12 days of paid sick leave a year, unbankable. (Q: Why 12? A: Because it makes the math easy.) You can use these days at any point during the year, but you aren't going to get more than 12 in a year. And if you don't use them all because you haven't been that sick, they don't roll over into the next year.

Now suppose your employer announces that they're going to restructure things a bit. Not a change in benefits, just purely administrative. Now, instead of up to 12 sick days a year, you get up to 1 sick day a month. You can use this day at any point during the month, but you aren't going to get more than 1 in a month. If you don't use it that month because you're not sick, it doesn't roll over into the next month.

That's less helpful, isn't it? We don't get sick every month, and we don't always get only one day worth of sick in a calendar month. Most months you don't need any sick days, some months you need two. It's less realistic, less fair, and rather arbitrary. And if your employer did work that way, wouldn't it be more tempting than it is now to call in sick one day during the last week of the month just because you're tired or you have a bit of a sniffle or you need a mental health day?

Allowing employees to bank sick leave over a career is better than having sick leave expire at the end of the year for exactly the same reason why allowing employees to use their 12 days of sick leave at any point during a year is better than limiting them to one a month. It is a direct logical extrapolation.