Okay, so the point of a percentage split is that each investment category grows and performs over time at a different rate with a different risk exposure. So with a 80/20 or a 60/30/10 or other similar split, why would you rebalance? Wouldn't that just throw off your balance more? If you get say, $1000/month and buy $800 of one fund and $200 of another, it'll just always be balanced, right? As it grows at different rates, you still have 80% of your principle in one fund and 20 in another. The different risk exposure and growth rates are what you're looking for. If the 80 grows a lot, isn't that what you want to happen? I just find it a bit confusing that you would routinely move money from an overperforming category to an overperforming one when they're already balanced perfectly.

To be clear, switching up your allocation when you're close to maturity still makes sense, but even then, shouldn't you rebalance based on your principal rather than your final balance?