The bridge key was a vital piece of any Einstein-Rosen bridge that wished to connect to the bridge network in 2033. The key provided the proper autorization for the bridge to connect; without it connecting to the network was impossible.
In 2033 Malte Kerzach recognized and stole the Tenant's Way bridge's key when he, Florence Brennan, and James Zanasiu Jr were threatened by members of the Stripper mafia. The key became an important leverage point during Cimarron's coup, and later she showed her gratitude to the three by granting them one free jump through the bridge when they wanted it. ("CS:Under-Covered")