What may be the source of confusion is that the Geneva convention requires wearing uniforms... to get the protections afforded by Geneva convention. If your troops violate that requirement, then that means that if they're captured without uniforms, the enemy is free to not fulfil the prisoner of war treatment required by Geneva conventions, but summarily execute all of them as spies; which was also often the practical consequence in WW2 if such troops were cought. A parricular example may be the trial after WW2 of Otto Scorzeny and other officers for Nazi troops wearing USA uniforms during Operation Greif in Battle of Bulge, where they were acquitted on the claimed charges of war crimes because these actions were considered by the court as 'legitimate ruse of war'.

If I recall correctly, masquerading as Red Cross could be a war crime, there are specific provisions for that, but the international treaties do not prohibit to masquerade as civilians or enemy troops, or to perform all kinds of other misinformation.

For most members in most militaries, it's a legal requirement set by their command to wear uniforms - but it's a requirement that the commanders can alter if they deem it necessary.