In Acts 1:4 Christ tells the apostles to tarry in Jerusalem until they are endued with power so that they can go become witnesses.
The word that is important in that whole passage is tarry To tarry is to wait, to be in a place of expectation of what you are to receive.
The most important reason why we tarry in prayer is to build capacity in us to contain what we are to receive. Without this, what we receive will just destroy us.
Because when the anointing lands on you, it either elevates or destroys.
The difference is the season of your tarrying for it.