Employers deduct a part of your salary as taxes on every paycheck (usually monthly). But how much they deduct is based on your tax profile which is created by the tax authorities. It’s typically based on expected yearly income. However, you are allowed (and encouraged) to update this number yourself.

So say you swap to a lower paid job, or work less overtime than you planned, you can change your expected annual income to reflect this in taxes paid. If you don’t however, then you effectively pay too much taxes as your employer is basing the taxes on too high numbers.

Another reason is that you may have tax deductions that only gets computed into the total when you fill out your full tax report. So if you didn’t bake those into your tax profile at the start of the year, you might get returns on your taxes (with interest).

You can also downplay how much taxes to pay through salaries, in which case you will owe back taxes when the full tax report is made.