A certain amount of inefficiency or slack is necessary buffer in any system to reduce brittleness. When a problem occurs, a system that is running with 50% slack can recover more easily than a system with 5% slack.

See Germany's rail network, where almost every time-slot is occupied by a train, and then one train is delayed, and the system collapses with nobody getting to their destination on time for the rest of the day, until the overnight buffer.

In queuing problems, queue length (which means latency) is inversely proportional to slack time. If a network link is running a 90% capacity, on average there are 10 packets queued up and a packet that arrives will have to wait for 10 packet transmission times. At 99%, 100. At 99.99%, 10000. And if you try to use exactly 100% of your network link, the expected queue length is infinity, and the expected latency is infinity, which will not occur in practice because sometimes it will exceed available memory and packets will be dropped, even though utilization never exceeded 99.9999...%.