Sonic Pi's randomness is deterministic so a 'random' pattern you like can be reproduced and performed

In Sonic Pi nothing is truly random: the same seed always yields the same sequence of ‘random’ values. Artists initially object, wanting different music on every run — but Sam Aaron argues what they really want is reproducibility. If you use randomization to generate a riff or drum break you like, you need to be able to hold, name, reuse and perform it; with true randomness it slips away ‘like trying to pick up water with your hands’. Determinism gives you that grip, and you can still get variety by making it non-deterministic on purpose — e.g. setting the random seed to the current time gives fresh music each run. This is a clear case of musical need driving API design.

Examples

A random-generated melody replays identically each time the loop restarts; setting the random seed to the current time at the start yields a different pattern per run.

Assessment

Why is true randomness a problem for performing with Sonic Pi? How does deterministic randomness give both reproducibility and variety, and what is the mechanism for varying between runs?