A humid environment will cause electronic failures in all kinds of electronic equipment.
A United States Air Force study attributes 20% of all electronic failures to corrosion on contacts and components, etc., even if they have approved coatings.
Humidity will also influence the resistance values of insulation materials. This will cause minute currents to flow where they shouldn't, triggering system check failures and in some case irreparable damage.
Lowering the Relative Humidity (RH) to a level around 45% will cut the number of malfunctions approx. in half, compared with ambient conditions of approx. 80% RH.
Last but not least, humid air itself can be a good conductor. In case of high power electronics such as radars and communication equipment a too humid environment can lead to severe arcing and even cause fires.
Also in radar systems, humidity can cause ionisation of the air inside wave guide tubes. In those cases the air inside the wave guides should be dehumidified to a very low dewpoint.