Every ILS approach must have at least one point at which one can cross-check altitude vs. distance to runway. Historically it was outer marker but nowadays more often it's DME fix (hence the proliferation of no-OM ILSes). This crosscheck is done to assure that your altimeters are working correctly, that you have set the correct QNH, and, most importantly, that the you have the correct glideslope guidance.
Discusssion about false glideslopes and lot of useful links here.