Sea to summit says:
Increases sleeping bag performance by up to 20°F (11°C) with dual-weight Thermolite fabric. Our regular 80 g/m² is combined with panels of 110g/m² Thermolite in the core body and foot bed areas adding extra warmth where most needed. The compact, shorter design shaves off excess weight while giving maximum warmth for torso and feet.
Sea to Summit's Reactor Plus Thermolite Liner sits midway between the 'Reactor' and 'Reactor Extreme'. Its still made from Du Pont Thermalite fabric (this is a hollow core fibre that provides maximum insulation for weight), but in the case of the Compact Plus, the fabric is zoned to provide more insulation where its needed.
If you want a liner to keep your bag clean then this must be a consideration as it offers an insulation upgrade at the same time.
Could be used on its own if camping in a warm climate.