> What could it possibly be doing?

If memory serves: point-symbol optimizations to make code calls into The objective-C libraries performant.

Under The hood, objective-C really does use strings to reference library functions. So when the OS is updated, the string interning table is updated and all of the installed software has to have its string to symbol caches rewritten to account for the new API.

There are almost certainly better ways to do this but if I understand correctly, Apple inherited the existing solution without putting enough thought into how it would scale and now changing the solution would break an indeterminate amount of software depending upon the current implementation.