Question

What is Buffer Size?

Answer 1

The buffer size is the set amount of latency allowed.  While every system and device has a certain amount of inherent latency, the buffer setting allows the user to minimize or maximize this amount.  In general, 512 samples or 11-12 milliseconds will provide optimal performance while not delaying the signal long enough to be detectable by the human ear.  As a rule of thumb: the higher the buffer size, the more stable a system will be; the lower the buffer size, the better the performance of VSTi’s, etc will be, but system stability will suffer.

Answer 2

Playing VSTi instruments live, you want a buffer size of 64 samples or smaller. As a piano player I can feel the delay above 64 samples.

Not everyone is capable of playing live VSTi though, and that's why not everyone needs low buffer sizes. But real musicians do.

Modern audio engines have multiple buffer sizes in parallel. We don't know the internals of the Studio One audio engine.

Not everything needs to be rendered in realtime. That's a wrong assumption from the past. Modern DAWs experiment with new algorithms.