What's the point of having ILS on runway signs?
These markings designate the hold-short point during different runway operations.

Runway hold-short points are marked by white text on a red background. They designate a point which any aircraft must not cross without an explicit clearance from ATC. Note that in the below picture, there are two markings:



Cat II or III ILS is used during low visibility operations. In the case of an auto-land, the autopilot literally lands the plane on the runway. To do this, the aircraft systems need a very precise signal from the ILS antenna. Any aircraft on the ground crossing the line of sight between the ILS antenna and the landing aircraft will interfere the signal, sending the landing plane off course.

When a plane is landing on Cat II or III, other planes on the ground need to stay further back the runway. This has an undesirable side effect - the airport capacity is reduced. If somebody is doing a visual approach on a clear day, you can move a bit closer.

This is what happened to a Singapore Airlines Boeing 777 when it used auto-land without informing ATC. Since no one is aware they're using auto-land, the standard hold-short procedure (rather than the low visibility procedure) applies. They drifted off the runway pretty much like a racecar!