A sealed plastic enclosure isn't the be-all-end-all. Moisture diffuses through plastics, slowly but surely. Eventually the humidity inside rises to match the average ambient humidity. If that level is high enough that, at the lowest ambient temperature, water condenses: the game is over, you've lost.
Potting isn't impermeable either, but it does take up space. Liquid water can't condense on a corrodable surface if there's no surface exposed!
Closed (but not sealed) enclosures are typical where dripping water is expected. Where a sealed enclosure will trap moisture, a vented one allows the humidity to equalize with ambient humidity. This doesn't prevent condensation -- ambient conditions can certainly become condensing -- but it does mean it's unlikely for humidity inside the box to be much greater than the ambient, day-to-day level. Add some conformal coating to take care of the occasional condensation, and you have a pretty good box.
This construction is probably typical of lighting fixtures, automotive components and so on.
IP67 and such -- sealed enclosures -- are used where water under pressure is expected. Pressure washing (often automotive and industrial), submersion and so on, are typical conditions. Condensation is usually less of a priority. All you need is a sealed box, and sealed connectors; a regular board can be placed inside. If long life and condensation is a concern, then it should be potted as well.
Tim