i am not sure at which stage do you refer this question for synthesis, placement, optimization etc..
But anyways at either stage this is important and really helps in closing the timing for a specific design.
Basically from my understanding and experience so far EDA tools work better under a path group structure. ideally under the hood all these paths are made under a each clock and the tool engine is made to work on these paths relating to a clock.
Now in a design there will usally be at max 20% of the paths which are critical to close timing for. Path grouping them together enables an engineer to make the tool focus on these paths, add more pessimism, weight etc and also enables that the tool engine look at these path with priority and fix the same.
I see that these are only advantages to doing this always; the disadvantage that i see is a bit of increase in runtime depending on the number of paths and its slack / clock freq plus engineers effort to create these path groups.
cheers!!