Imagine you run a multitude of nodes. And your primary validator have a power shortage. If you are offline, you get punished taking what you would have earnt.

You try to avoid this and build a backup system. This one turns ON when your primary goes offline.

You keep operating as usual without losing rewards.

Suddenly the lights comes ON again and now the network detects two instances of the same validator. But not the same content.

The network sees you as an hostile entity and punish you for it, slashing your eths holding and kicking you out of the network.

Now, the risk of trying to backup your nodes might not compensate for the chance of getting slashed.

Thats where Fault Tolerating Staking (FTS) comes to play.