The shock troops in WWII where offensives troops with the mission to create gap in enemy lines. That is why they where equipped with body armor which would little point in wereing if they where used in defensive positions.
According to what i searched in google, the meaning of Shock Troops is the
following :
Shock troops or assault troops are formations created to lead an attack. They are often better trained and equipped than other infantry, and expected to take heavy casualties even in successful operations.
"Shock troop" is a calque, a loose translation of the German word Stoßtrupp. Military units which contain assault troops are typically organized for mobility with the intention that they will penetrate enemy defences and attack into the enemy's vulnerable rear areas. Any specialized, elite unit formed to fight an engagement via overwhelming assault (usually) would be considered shock troops, as opposed to a "special forces" or commando style units (intended mostly for covert operations). Both types of units could fight behind enemy lines, by surprise if required, however.
Although the term "shock troop" became popular in the 20th century, the concept is not a new one, such as the utilization by Western European armies of the forlorn hope. Presently, the term is rarely used as the strategic concepts behind it have become standard contemporary military thinking.
With the same logic, Rangers can also be considered Shock Troops as they are speciliazed in frontline assaults. In Ancient Greece, the Greek forces that fought in the battle of Marathon were also considered Shock Troops, because they led the attack. They caught the Persian army by surprise.