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Examples of why declaring data in a class as personal is important?

I understand that only a class can access the data, therefore it is “safer” and what is not, but I really don’t understand why this is such a big deal. Perhaps this is due to the fact that I did not make the programs complex enough so that the data could accidentally change, but it is a little confusing when teaching classes and says that making things private is important because it is “safer” when the only time when I changed the data in the program - this is when I clearly wanted to. Can someone give some examples where the data would be inadvertently changed if the data were not private?

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