They've also had their currency pegged to the Euro since 2005 (https://en.wikipedia.org/wiki/European_Exchange_Rate_Mechani...), so in a sense their currency has already for years been treated commercially as merely an alias unit for the Euro, like one converts lbs and kg at a fixed rate. The main difference with actually adopting the Euro is: 1) harder to change your mind later, since un-adopting the Euro is harder than abandoning a currency peg; and 2) easier for tourists and cross-border business, since the need for currency conversion & the associated fees is removed. It might also simplify cross-border business accounting, since you'd no longer have assets & liabilities in different currencies (even if pegged ones), though I know less about that.
Denmark is in a similar situation: the Danish crown is pegged to the Euro since 1999, so in practice Denmark has no real scope for independent monetary policy. Public opinion is still against joining the Euro for a mixture of nationalistic reasons (people like the currency as a symbol of sovereignty) and value put on having an exit option if things get too dicey (one exit option is to peg to the Swedish crown instead; another is to fully float).
I can read your comment as pointing this out already, but an important third option is to stay pegged to the Euro, but at a different exchange rate. That's a very large freedom that you lose when you "formalize" a Euro peg by just switching over to the Euro.
For example:
Year 2024: Our currency is pegged at 3.28 to the Euro.
Year 2027: Euros have become too valuable; our peg is now 4.1 to the Euro.
Good point, that's definitely another ability a country in ERM retains (though if done unilaterally it would still constitute pulling out of ERM as a treaty). That's what China does with their unilateral and occasionally revised peg to the dollar, for example.
I'm not entirely sure what the results would be, but you do lose some of the current benefits if people start believing the exchange rate is no longer really fixed. Currently the large amount of Danish-German cross-border trade effectively ignores currency exchange risk, because they assume the EUR-DKK peg as formalized through ERM is solid enough that it won't change in the forseeable future. So it's "safe" to have liabilities in DKK and revenue in EUR or vice-versa without taking any particular measures to hedge your risk. That was part of the goal of both the Euro and ERM, since being able to ignore exchange risk makes it easier for businesses to treat Europe as a single market. Once you change it once, then it might be a while again before businesses feel comfortable relying on the new peg.
Denmark is in a similar situation: the Danish crown is pegged to the Euro since 1999, so in practice Denmark has no real scope for independent monetary policy. Public opinion is still against joining the Euro for a mixture of nationalistic reasons (people like the currency as a symbol of sovereignty) and value put on having an exit option if things get too dicey (one exit option is to peg to the Swedish crown instead; another is to fully float).