Pretty simple, let me explain it using the transpose effect as an example:
Song 1: Preset A - Transpose 0, Preset B - Transpose 0
Lock transpose parameter: -2
Song 2: Preset A - Transpose -2, Preset B - Transpose -2
This feature would allow to switch presets maintaining one parameter locked, especially useful if you need to play in different tunings during a gig but only have 1 guitar. Otherwise this could be fixed with the “banks of presets” function, in order to clone presets with different transpose settings into dedicated banks.