“But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.”  Acts 1:8

Jesus thought the disciples needed to receive the Holy Ghost to be effective witnesses, and he highlighted the fact that the Holy Ghost was to endue them with power.
Power is the ability to effect a change, this means he thought without that ability to effect changes they would be ineffective witnesses.
When he sent the disciples out during his sojourn on the earth, he told them to preach the good news and he endued them with power:
“And when he had called unto him his twelve disciples, he gave them power against unclean spirits, to cast them out, and to heal all manner of sickness and all manner of disease.”  Matthew 10:1
After which he clearly expatiated what they should expect and what changes they should effect:
“Heal the sick, cleanse the lepers, raise the dead, cast out devils: freely ye have received, freely give.”  Matthew 10:8
He thought when they got to an environment, and there was sickness or loss, they could effect changes and cause health and restoration, just like he did:
“How God anointed Jesus of Nazareth with the Holy Ghost and with power: who went about doing good, and healing all that were oppressed of the devil; for God was with him.”  Acts 10:38