Joining ground between ESP32 and LED Strip from single power supply
Ground is defined as a 0V reference.  When supply current is returned to ground, it can cause a shift in the voltage along the wire resistance.  Worse yet with PWM dimming and wire inductance there can be a voltage variation from  wire inductance (Vac=LdI/dt) 

Thus it is common to use wide or braided grounds to keep the voltage constant with low effective series resistance (ESR) and inductance (ESL).  

This is important  both to reduce audio noise and digital communication errors.