If someone owes another person $100, but for some number of years the person who was owed money fails to make any effort to collect on that debt, the debt becomes legally uncollectable no matter how solid the evidence may be that the money was owed and never paid. While there are specific rules which change the statutes of limitations for some types of debt, I am unaware of any that would be applicable to traffic tickets or other civil fines.
Were it not for such statutes of limitations, anyone who ever paid any sort of debt would have to keep forever the evidence of such payment. An N-year statute of limitations gives one the freedom to dispose of receipts that are more than N years old, without fear of being compelled to produce them.
Thanks - there's something 'fair' about the above... that's nice.