Jesus is saying that it’s to their advantage that the Holy Spirit comes, so that they can be filled and changed.
The Holy Spirit came and filled all of the people in the upper room, including Mary (which means she’s a pentecostal).
They were praying in the upper, in one accord, waiting for the Holy Spirit. That was their one purpose.
They weren’t there to hear a band, they weren’t there to dig a well, they were there to seek the Holy Spirit so they could be a witness for the Kingdom.
When you get filled with the Holy Ghost, you’ll get vision. The ultimate goal of every Christian is to be a witness.