Entropy
Computer programs are deterministic: given the same input, they'll produce the same output. Even pseudorandom generators are perfectly predictable, and hence utterly boring.
The solution is to ingest a little entropy every now and then: arbitrary bits drawn from some chaotic, unpredictable process. Entropy makes our crypto keys unguessable, lets us re-seed our PRNGs, lifts our SGDs from their local minima, and reinvigorates the topiary our MCTS. We can revisit old data with fresh eyes, to spot new patterns; and, crucially, we gain some resilience against pathological/adversarial inputs.
There are many sources of entropy, depending on taste. Some make do with a slow trickle from the lower bits of the system clock; others indulge from the firehose of a dedicated metastable quantum oscillator circuit. The truly paranoid insist that all internal sources are suspect; it's not uncommon for the radio telescope task queue to be filled up to 20% with their patient observations of distant pulsars.