[asveikau]

I can think of one use.

Let's say you have two pointers that are sometimes unique and sometimes aliases. Maybe they mean semantically different things but they happen to be the same for some cases, and different in others. They always are on the heap. You want to clean them up when your function exits, freeing them both, or once if they are not unique.

    free(p);
    if (p != q)
       free(q);

Believe it or not I have written something like this, although with integer file descriptors being closed rather than heap buffers freed. eg. Maybe some fds are passed for both reading or writing, or sometimes you have a unique fd for each, but it all needs to hit close(2).

To exist within the standard I guess you need to do the comparison first:

    if (p != q)
       free(p);
    free(q);

Edit: ah, but you just said "cross-object pointer comparisons are UB". I can't see a good reason for that either, but I do suppose it might make some architecture's non-linear pointer representation work better.

[jcranmer]

For some reason, I thought equality and inequality pointer comparisons required pointers to be in the same object. It's actually only the relational operators that are undefined if not in the same object. (Although I believe most compilers will treat them as unspecified instead of undefined).