No point havin em side by side
not always
if you have a long chain of noisy effects, you could use one before them (to keep the guitar noise from getting amplified) and another at the end of the chain
in my experience, most of the noise comes from electro/electronic devices in the room (including some lamps, fans, PC monitors) interfering with the guitar pickups
it just gets amplified by the pedals (especially overdrives, boosters and equalizers) and the amp
most outlets here are hardly grounded, so supply powered pedals tend to add some noise (including the noise gate itself)