See the big picture
You can see the interrelationships between the web tier, application tier, database servers, message queues, external cloud services etc. in real time while you're testing rather than being focused on the metrics for each tier individually.
Drill down to the code level
Load testing with APM tools is being able to drill down to the stack trace level and identify the calls that are the most problematic.
An agile mindset and continuous delivery capabilities, can enable you to do rapid test and fix cycles during testing, often multiple times times in a day.
Testing becomes more collaborative because it's easier to share the performance data in real time via the Application Performance Management tool, and discussions become more evidence-based.