Well, here's what I've learned by my testing.
Let's say I have 2 waypoints and I set up waypoint 1 with a waypoint action to move the gimbal -10 (forward) and waypoint 2 to set the gimbal -90 (down).
When the drone hits waypoint 1 the gimbal will go to -10 quickly. When the drone hits waypoint 2 it will go to -90 quickly.
Now, let's say I don't use waypoint actions, I use an interpolate of -10 for waypoint 1 and an interpolate for waypoint 2 of -90.
This time when the drone hits waypoint 1 the gimbal moves to -10 but on it's way to waypoint 2 it starts moving to -90.
If the distance between waypoint 1 and waypoint 2 is 100 feet, it would happen fairly quickly.
If the distance between waypoint and waypoint 2 is 2000 feet, it would happen very gradually.
Imagine flying over a lake and you interpolate between -90 (down) looking at the water and you interpolate to a -10 (horizon) shot, and the distance between the two waypoints was 500', that might look cool moving the view from the lake below to the horizon.
I hope I explained that correctly.
Once I learned about it, I realized I would not use interpolate regularly, but I'd also like to hear other scenarios where people use it.