The difference between a flame graph and a trace visualization is that a flame graph is a aggregate/summary visualization. It helps visualize total runtime attributed to functions.
It is like the difference between seeing the mean of a distribution and seeing a plot of every datapoint in the distribution. They are useful for different purposes.
An example of how you might use it in conjunction with a trace visualizer is that you would select a time span in a trace and generate a flame graph for the selection. This would show you which functions and call stacks were responsible for most of the execution time in the selection. You would then use that to find one of those call stacks in the trace to examine how they execute to see if it makes sense.
It is like the difference between seeing the mean of a distribution and seeing a plot of every datapoint in the distribution. They are useful for different purposes.
An example of how you might use it in conjunction with a trace visualizer is that you would select a time span in a trace and generate a flame graph for the selection. This would show you which functions and call stacks were responsible for most of the execution time in the selection. You would then use that to find one of those call stacks in the trace to examine how they execute to see if it makes sense.