After the death of the Lord the disciples thought like everything is over with these feelings in heart they hid themselves but St. John says that the Lord stood among them. Resurrected Jesus was different from the historical Jesus. He through the power of the Holy Spirit was with and amidst the apostles.
He greets them with the peace of the Holy Spirit. The apostles for a moment were happy and joyful to see the Lord Jesus standing among them. The Lord did not want to fill them up with the short happiness and joy but the everlasting one with the presence of the Holy Spirit.
Then the Lords sends them to be witness of the good news in the world as He himself did it. In order to be witness the Holy Spirit sends them to the entire world. To be the witnesses of the resurrected Lord the apostles did need the power and strength of the Holy spirit.
Jesus not only spoke about the Holy spirit with the apostles but He breathed on them that they may receive the Holy spirit.