Or 'across the pond' via the North Atlantic Track system or up/down the Tango route system to the Canaries from Ireland. It means that you can keep 'managed speed' throughout most of the flight if the Mach Number is defined only along a particular section of an airway (ii.e. T16) or along a NAT track.
It also it should give you a better MCDU estimate of the fuel at the destination because the aircraft will be predicting flying managed speed before, constant Mach on the section of airway and managed after.