The steward controlled access to the royal stores and for this purpose received big wooden keys which were carried upon the shoulder.
There is also a symbolic meaning to this verse. The steward had great power—he could give people access to the king but could also restrict access. He could make important decisions, and he exerted influence over the king. It is made clear that Eliakim was no usurper—God gave him his power and authority. The keys were entrusted to Eliakim as to Peter and the other apostles in the New Testament (Matthew 16:19).
Although Christ alone is the Head and King of the church and has no need of administrators such as Eliakim, the king in the Old Testament time was merely a type of the Christ and not an equivalent. In Judah, in the days of Isaiah, there was a need for such an office, and God invested Eliakim with the power to use his office in the right way, unlike Shebna before him.
22 And I will place on his shoulder the key of the house of David. He shall open, and none shall shut; and he shall shut, and none shall open.