If shares have a good year, a Balanced portfolio that started at 55% shares might end the year at 62%. It now carries more risk than the level you agreed.
Rebalancing sells a little of what has grown and buys what has fallen behind, so the mix goes back to where it should be. It can feel odd to sell the winners, but it keeps the risk where you chose it.
We rebalance in April and October, and earlier only if a mix drifts more than five points from its target.