It is sensible. First, deductibles are applied to the bands, like everywhere else. This calculation is done over your yearly salary, in March of the following year. You then pay income tax in August.
However, in order to minimize the amount due in August, the government captures an estimate of your income tax every month (before you get paid).
It is this estimate that is designed to be simple, and applies deductibles to the whole salary. There's no harm: If it happens to capture too much, you'll see your money back next August.
> I still don’t understand the benefit over a progressive system.
Simplicity and personal data protection. In order to capture tax in advance with perfect estimates, employers would need to know your total income and deductible expenses. With the simplistic system, they just need to know how much they're paying you.
However, in order to minimize the amount due in August, the government captures an estimate of your income tax every month (before you get paid).
It is this estimate that is designed to be simple, and applies deductibles to the whole salary. There's no harm: If it happens to capture too much, you'll see your money back next August.