to prevent trasportation of military hardware by sea, which is the most efficient way of trasnporting weaponary and gear that can be used to make weaponry. Gilad Shalit was not the reason to put up the blokade and returning him home is therefore not the reason to remove it. The blokade should stay in place as long as Hamas does not recognise Israel's right to exist a the state of the Jewish nation.
U.S. confirms ISIS computer expert killed in air strike (Reuters)
from the article: Hamas: Israel pledged to lift Gaza blockade as part of Shalit swap deal