You can also view a visual representation of the callgraph if you prefer this more detailed profiling to the timeline profiler. By clicking on "Callgraph" in the trace you can see the callgraph from top-down following the hot path. Or you can click on the small call graph symbols in the call details panel to see the callgraph with both children and parent functions for the selected call.
When you are debugging performance problems a full callgraph can give you more detailed insights into every function call of your PHP requests.
Because the Callgraph Profiler has a noticable overhead over the Timeline Profiler, it is only available for Developer Triggered traces by default.