It's not that hard to trace duration of a given function call with dtrace without such annotations, but they make it much easier to discover where in a large codebase to look, providing a form of documentation. With perf they make life much easier. I should add some more to make it easier to analyse relation extension contention on indexes and the heap, instrument btree index ops like page splits, instrument heavyweight locking (beyond LOCK_WAIT_START), etc. They're also handy for gdb - https://sourceware.org/gdb/onlinedocs/gdb/Static-Probe-Points.html - also handy for gdb's "info probes".
BTW, we might want to instrument the pgstat_ counter calls and pgstat_report_wait_start / pgstat_report_wait_end, but it's easy enough to use dynamic tracepoints for those so I haven't yet. Maybe even just document them as points of interest.
