If you are VRF the use is moot as you will visually have the runway in sight. Flying IFR, In IMC on a ILS approach is when the pilot makes the most use of them
The outer marker is normally located 3 nm;to 4.5 nm from the threshold except that, where this distance is not practical, the outer marker may be located between further from the threshold. The modulation is repeated Morse-style dashes of a 400 Hz tone. The cockpit indicator is a blue lamp that flashes in unison with the received audio code. The purpose of this beacon is to provide height, distance and equipment functioning checks to aircraft on intermediate and final approach. In the United States, a NDB is often combined with the outer marker beacon in the ILS approach (called a Locator Outer Marker, or LOM); in Canada, low-powered NDBs have replaced marker beacons entirely.
Middle marker
The middle marker should be located so as to indicate, in low visibility conditions, the missed approach point, and the point that visual contact with the runway is imminent, ideally at a distance of approximately 3,500 ft from the threshold. It is modulated with a 1.3 kHz tone as alternating Morse-style dots and dashes at the rate of two per second. The cockpit indicator is an amber lamp that flashes in unison with the received audio code. Middle markers are no longer required in the United States so many of them are being decommissioned.
Inner marker
The inner marker, when installed, shall be located so as to indicate in low visibility conditions the imminence of arrival at the runway threshold. This is typically the position of an aircraft on the ILS as it reaches Category II minima. Ideally at a distance of approximately 1,000 ft from the threshold. The modulation is Morse-style dots at 3 kHz. The cockpit indicator is a white lamp that flashes in unison with the received audio code.