In speaking to the Apostles after the Last Supper Jesus brought up some things for the first time. He spoke to them about going to prepare a place for them. He spoke about sending them the Spirit, saying they would be better off with the invisible presence of the Spirit than they were with his physical presence.
But, for all the new things Jesus told the Apostles that night, he still had to hold back on telling them certain truths that would be necessary for the Church in years to come. (He said they were not ready to hear those things.) He had to leave it to the Spirit to teach them those further lessons when the Apostles or their successors would be mentally prepared to receive and understand them.
The Church believes that it is still possible to receive inspiration from the Spirit to deal with new problems that arise in these later centuries.
For instance, in 1632 the Church still believed that everything in the Bible was literally true, so they thought Galileo was in heresy for doubting the Biblical account of the sun standing still while God’s champion finished off some foe. But by 1964 the Church was saying that the Holy Spirit was leading them to accept the modern scholarship that held for the Bible’s containing myths and folk legends.
In our rapidly changing world with un-thought of innovations coming every year the Church must have constant recourse to the Holy Spirit to help it know what God wants of it.