See bitcoindevkit/bdk#640 (comment)
The idea is that if I have 10BTC in my wallet which are confirmed, but I'll be able to spend it only in a few months, then maybe get_balance should reflect that, and count the coins as locked or something.
This would mean passing a policy to get_balance so that it knows with which descriptor policy we would like to spend our coins.
Since this is a pretty advanced feature, maybe we should consider adding another API (get_balance_with_policy or something), so that normal users can just use get_balance and be fine.