In Europe / North Atlantic it is used when in MNPS Airspace. The theory is that modern navigation systems are so accurate all aircraft will be flying down the same very narrow corridor whereas with less accurate nav systems aircraft would be a bit mor spaced out. (If I remember correctly and airways in 10Nm wide to accommodate this?) Nowadays aircraft could theoretically, and often do, fly within a corridor just a number of metre wide.
With SLOP (where I am based) you offset right by 0, 1 or 2 miles. This in theory means a third of the aircraft are on the centreline of the airway, a third of the aircraft are 1Nm right of the centreline, and a third of the aircraft are 2Nm right of the centreline.
The same applies in the opposite direction, so you now have aircraft spread across 4 miles.
This means less chance of a collision, and provides a safety margin.
NATS (Oceanic) did an excellent video that explains it - I'll see if I can find an online copy.