We coat all our circuits with a conformal coating to protect the leads and contacts to protect them from environmental issues. At the speeds the circuits are running, any two neighboring leads act as capacitors. If a piece of dust or a drop of moisture lands on them they might as well be a short circuit even though they are not touching. If that happens, at best, you have incorrect results, at worst your circuit is destroyed.
The only way we can assure a known dielectric between the leads is to coat them with something that has well defined and constant properties over the entire temperature and humidity range. The added step costs money but it is well worth it for the manufacturer and the customer.