HJ,
'cos you are using it in association with the glide path signal. It's an admitedly basic triangulation of marker distance and GP angle.
If for example you receive an middle marker signal at 4000ft above airfield elevation when established on the GP, you know you have captured a false beam.
Selecting LO changes the reception cone from something like 2nm diameter to approx 1/2 to 3/4 nm.
Marker signals are also used in some autopilot systems to start a gain reduction program which prevents LOC and GP over shoot due to beam convergence.