there is a school of thought that rupturing the membranes speeds up labour, therefore freeing up the labour bed sooner. However the most recent research has refuted this, so it is no longer common in many units to break the waters just for the sake of it.

Breaking the waters can stimulate contractions if they are weak or non existent though, hence it is used as part of the induction process. It may be that your friend was not yet in established labour, and they decided to break her waters to get her into labour. It is also used if labour has stalled (for example if a woman is stuck at 9cms, breaking the waters will bring the baby's head down onto the cervix and help it dilate that final cm). Or it could be that they are following guidelines based on older research. As a general rule though, if induction or augmention of labour with a drip is being considered, the waters will be broken first as the drugs are less effective if the membranes are still intact and sometimes breaking the waters will be enough on its own. But in a normal labour it is often not necessary.