All the numbers have secondary forms with extra strokes. (In Chinese too.) It's originally an anti-fraud/legibility technique. But they sometimes get used in

All the numbers have secondary forms with extra strokes. (In Chinese too.) It's originally an anti-fraud/legibility technique. But they sometimes get used in formal/legal contexts just because. (One stroke turns 十 10 into 千 1000 otherwise, which is problematic on something like a cheque.)