Balance comes from the body, not the hands. Here is why we take the reins away before we give them back.
A beginner given reins on day one will use them for balance, because that is what a nervous hand does. Six weeks later the same rider is still gripping, and undoing it takes a term.
On the lunge with no reins, the seat has to do the work. It feels slower for the first fortnight and then it is not slow at all: riders who start this way reach a controlled canter roughly a term ahead of riders who started with a contact they were leaning on.