The idea is: keep extra of the important things, so that if something goes wrong, you can still get by.
A person's glasses, the elastic band of suspenders, a car's spare tire, easily lost earphones, a goddess's simp, a player's fish — all are typical examples of redundancy backup. If the main one is gone, the other one or the others can still suffice.
The redundancy backup model is not simply the concept of backup; it also has the following three characteristics:
It also participates in the system's normal operation, rather than necessarily staying dormant and only being activated after a component fails.
It does not necessarily look the same as the main working component, but it can serve a function that achieves the same end goal.
It often adds extra cost to the system, though the amount depends on how well the backup is designed.
How can we apply this to our lives? If you treat household income as a system, your side business and your wife's income are redundancy backups for the household income system. That way, if you lose your job, as long as your wife can still earn money or your side business can make money, you may take some flak during this period, but at least you can still get by.
But of course you can't add backups without regard to cost. Different systems have different consequences when things go wrong, so it depends on the situation. We need to consider whether the cost is less than the potential loss; if the loss is major, you need a backup.
When the maximum loss if an accident occurs × the probability of the accident is greater than the cost of the backup investment, you can back up.