At some point in their past they experienced intense inflation, where the value of their currency went down very quickly relative to the price of buying things – situations where e.g. something that cost one unit yesterday might cost four today and twelve tomorrow. Some countries where this happened have since issued new currencies with better-looking numbers (e.g. saying ‘1 new unit is worth 1000 of the old unit’), but some haven’t felt the need.
It’s the same reason that in the past you could actually buy small things for $.25 USD, and now that much money is effectively worthless. It’s just happened more and faster for some other currencies.
A famous example of this is Germany in the 1920s, where money ended up so worthless that people were burning banknotes as fuel. The worst historical example of this was the Hungarian pengő after WW2, which was replaced by the forint in 1946 at a rate of 400,000,000,000,000,000,000,000,000,000 (400 octillion; 4×10^(29) ) pengő to one forint. (As of today, the exchange rate is 395 HUF for 1 USD, meaning that 1 USD is currently worth 1.58×10^(34) old Hungarian pengő.)
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