Adam Clarke on Acts 23:11

Be of good cheer, Paul - It is no wonder if, with all these trials and difficulties, St. Paul was much dejected in mind; and especially as he had not any direct intimation from God what the end of the present trials would be: to comfort him and strengthen his faith, God gave him this vision.

So must thou bear witness also at Rome - This was pleasing intelligence to Paul, who had long desired to see that city, and preach the Gospel of Christ there. He appears to have had an intimation that he should see it; but how, he could not tell; and this vision satisfied him that he should be sent thither by God himself. This would settle every fear and scruple concerning the issue of the present persecution.