Left alone, your careful 70/30 portfolio drifts into something riskier than you chose. Rebalancing quietly drags it back — and forces you to buy low.
You set a sensible mix — say 70% stocks, 30% bonds. Then the market moves, and a year later you're at 80/20 without changing a thing. Your portfolio drifted into more risk than you picked. Rebalancing is how you fix that.
— Educational, not financial advice.