Espionage/sabotage has been a legitimate part of warfare since mankind. The same thing as ambushes, raids, etc and so on. The idea of going behind enemy lines without wearing something to show who you actually are is a best way to destroy the enemy's ability to wage war, along with aerial bombardment of military installations behind enemy lines. Enemy troops won't know who you actually are so they let your pass by, thus allowing you to go on a mission to carry out espionage/sabotage missions.

For espionage, you go behind enemy lines without proper insignia in order to obtain information about the movement/concentration of enemy troops then relay it on your own side so your own commanders truly know where the enemy were. Therefore, you saved the lives of your own men by knowing where the enemy were so they could assess and make a drop on the enemy.

Sabotage is the same thing as well. You go behind enemy lines without wearing proper insignia and tried to sabotage enemy targets by cutting off telephones, blowing up factories, bridges, and other targets. That way, enemy troops won't know about it and/or who actually did these things when you escaped back to friendly lines.