The Spy's supreme ability to disguise himself prooves him the best Secret Agent around. When wearing the uniform, the Spy can talk to enemies and distract them. The Spy will lose the uniform if spotted doing something 'not right'. e.g. Killing a German soldier.
When he is out of sight, he can put the uniform back on again. His ability to fool the enemies is determining for your success. But beware of the SS Agents since they cannot be fooled, not even by Frenchy.
TIP: You will be able to use the weapons of the enemy you took the uniform of.