Pentecost was a Jewish feast where Israelites from all over the world would return to the homeland.  This was the perfect time for the Holy Spirit to come down, as prophesied in Joel, and permanently dwell in the hearts of all believers, because He used this as an opportunity to have the apostles preach to these visitors, even allowing the group to preach in the native languages of the various audience members without ever taking a class.  From there, hundreds were saved and the church was officially born.

What additional result happened to the disciples?

-They grew wings-

-They were no longer scared, but were bold for Christ-

-They fell on the ground, laughing-

-They barked like dogs-