An ILS consists, at its most basic, of two components: a localiser (to provide lateral guidance) and a glideslope (to provide vertical guidance). The two are typically used together, for an ILS approach, but the localiser can also be used on its own, for a localiser-only, or simply localiser (LOC for short), approach (for instance, if the glideslope is inoperative or nonexistent, the aircraft lacks a functioning glideslope receiver, or the pilot is shooting a back-course approach). For a localiser-only approach, the pilot uses a specified series of navigational fixes to mark off each segment of the approach; each of these segments has a specified minimum safe altitude (which is shown on the approach plate), and the pilot uses this information, and their onboard baroaltimeter, to maintain terrain clearance during the approach.
While localiser-sans-glideslope approaches are quite common, there are, to the best of my knowledge (feel free to correct me if I'm mistaken here), no examples of the reverse: a glideslope-sans-localiser approach, where the pilot uses the glideslope for vertical guidance but relies on ground- or satellite-based navigation fixes for lateral guidance.
What allows the use for an approach of a localiser without a glideslope, but not a glideslope without a localiser?