The mono core and stereo spread principle
The kick, bass, and vocal live in the center because they carry the low-frequency content that defines the song's power and clarity. Panning frequencies below around 200 Hz to one side of the stereo field creates an unbalanced, unstable foundation. Your listener's brain expects weight in the middle, so centering these elements lets higher frequencies spread around them without losing groundedness.
Leaving the center clear for the mix's core elements actually makes the entire stereo image feel wider, because the ear compares the relative space of centered vs panned elements.
Guitars, keys, and effects in wide stereo
Guitars and keyboards can live anywhere from center to 40-60% panned left and right. Panning them apart creates separation without sacrificing definition. A slight offset is often better than hard left/right, which can sound unnatural in headphones or small speakers.
Effects like reverb, delay, and modulation can be panned wider than the source instruments, anchoring them to the stereo image while keeping them clearly secondary. A synth pad panned at 40% left with a reverb send panned at 80% left pushes the effect into space while keeping the core sound grounded.