The reason for the segments is the requirements.
If you require something, you have to be specific.
In an engine out situation you need to climb, accelerate and clean, and authorities require minimum climb gradients for each part o the process:
1 when you still have the gear down,
2 when you have it up already,
3 when you accelerate in level flight and clean up (the acceleration rate required is the equivalent to a climb gradient requierement), and
4 when you are clean and climbing with MCT
For each stage they make a different requirement, hence the 4 segments
This is for climbing requirements, not for obstacle. For obstacle you have to deal with the net flight path, the DER, and all that stuff.