Reminder:

Jesus had been resurrected. Jesus had died... crucified and buried!
3 days after his death and burial, He came back to life! They, the Apostles had been wih Him..eaten with Him!

Act 1:4-5 ISV 4 While he was meeting with them, he ordered them, “Don’t leave Jerusalem. Instead, wait for what the Father has promised, about which you heard me speak, 5 because John baptized with water, but you will be baptized with the Holy Spirit a few days from now.”

And here the Lord Jesus the Christ tells them not to leave Jerusalem until the Holy Spirit had come. The Holy Spirit would come with power, which would make them witnesses.

But were they not witnesses of the Resurrected Lord already??
Was He not standing here Alive!!! Were they not seeing Him and eating with Him????
Who is a witness..is it not related to testifying of what you have actually seen?? They had seen Him, ALIVE!!!!!!

What more did they need?