const volatile is used a lot when doing HW programming. Const will prevent your code from editing it and volatile prevents the compiler from making assumptions. For example reading from a read only MMIO region. Hardware might change the value hence volatile but you can't because it's read only so marking it as const allows the compiler to catch it instead of allowing you to try and fail.

AFAIK when it’s not volatile, the compiler can place it into read-only data segment

True, but preventing that is merely a side effect of the volatile qualifier when applied to any random variable. The reason for volatile's existence is that some memory is changed by the underlying hardware, or by an external process, or by the act of accessing it.

The qualifier was a necessary addition to C in order to support such cases, which you might not encounter if you mainly deal with application code, but you'll see quite a bit in domains like hardware drivers and embedded systems.

A const volatile variable is simply one of these that doesn't accept explicit writes. A sensor output, for example.

The very notion of "less of a UB" is against the concept of UB. If you have an UB in your program, all guarantees are out of the window.