Intellirepeaters are for "radio holes" or difficult reception areas. They fill in the holes by communicating with units in the immediate vicinity. If the signal is stronger on the IR site then the mobile will switch to IR sites from one of the Simulcast layers. IR sites do not repeat everything from the S1/S2 layers unless the IR site is actually serving a mobile unit in the area on the TG the mobile is tuned to. In other words any TG can appear on an IR site but only if mobiles are in the area and using that TG. As the mobile moves out of the IR area (and no other mobile is in the area on that TG) then the TG will stop transmitting on the IR site.
They're great if you're near one as they will go active if something is going in the immediate vicinity. FYI ... "immediate vicinity" varies in size depending the intended coverage area, terrain, etc. So, IR sites can "fill in" a large area, if necessary.