One reason for export restrictions on your currency can be to prevent foreign government agencies (CIA...) from building up stockpiles to be used in covert operations.
Less to do with economics than politics of course, unless that stockpile is then dumped on your market and used to disrupt the economy through an oversupply of funds (many countries try to regulate the value of their currency by controlling the amount of it that exists in circulation).
To have a large enough effect to seriously damage an economy however would require such large amounts of cash that it's extremely unlikely any foreign agency would be able to acquire that much in a reasonable amount of time.
But using it to bribe or otherwise pay your citizens in key positions to act as their agents is a definite risk, and if those citizens are in positions where they can cause economic disruptions even a smaller amount of money can cause economic trouble (think of the theoretical foreign agency bribing key people in the Iranian oil industry to sabotage all their export terminals to fail at the same time for example).
Combined with using your currency export ban as a trap for foreign currency (foreigners now cannot buy your currency abroad, so the only way visitors to your country can get your currency is by buying it using their own, more valuable, currency at your government controlled banks) and you have a powerful incentive for some countries to impose export bans on their currency.
And that last incentive is very real, and the main reason the USSR and others (including many of its former constituent states) have such restrictions, it's a big source of hard currency for them (in fact some such countries even require foreign visitors to exchange a minimum amount for every day they stay, an amount higher than they're likely to need for their minimum expenses of lodging, food, and transport, as an incentive for foreigners to spend big on souvenirs).