A radio altimeter is an excellent reminder that you have reached your minimum
Only at airports whose approach terrain has been suitably prepared for radalt-based approaches (Cat3 only I think).
Those approaches switch the lateral autopilot guidance to the radalt at about 150ft which is well before the start of the runway.
Another use is as a primitive ground proximity warning system.
Very primitive; the beam points only slightly forward, so it will warn only against gradually rising ground.
Look up that seminal Mt Erebus crash. They had a radalt. It gave them just a few secs' warning. They pulled up to max, but too late. The only GPWS which actually works is a GPS plus a terrain map; mandatory in all transport planes over X seats.
Of course, modern GPS based avionics have a much better terrain warning built into them, but not every little club aircraft has them.
Yes; a £1000 Garmin 496, with its audio output wired to the intercom, is a vastly better "GPWS" than a £10000 KRA10 installation.
After that I tried to see whether it was possible to land in zero zero conditions using a radar alt for the flair.
Tried the experiment on an ILS in VMC with a safety pilot and yes I feel confident in an emergency its quite feasible using a rad alt in a Seneca Twin.
A reasonable autopilot will do that for you too, on an ILS. It will take you to the runway, and you will definitely have enough surface visibility at that point no matter how thick the fog is.
Based on cloud OVC, SEP IMCR PPL, we reakon 1800 feet over land
That seems OTT, because you could fly below the cloud, VFR, well above the MSA
