Good tension between partners in crucial to good dancing in ballroom. Without tension, partners would have no way to communicate with each other. The lead controls the follow by pushing of pulling them, either by the shoulder in closed position or by the arms in open position. But in order for the lead to push the follow, the follow must have resistance or tension. This does not mean the follow should fight the lead and refuse to move, but a little independence never hurt anyone.
In this video Randal Barnes and Rachel Holland demonstrate why have good tension is so important.