Well it’s self-explanatory. Say you have a vector that points in a direction, you use ‘Make Rot From X’ and apply that rotator to an object in World-Space. The object’s X-axis will now be facing the same direction that the vector was pointing.
You can use Make Rot From XY or other multiple angles, to force orthogonality and therefore get completely correct rotators, which is also important in a lot of situations. You COULD use Make from Axis and Angle, but that’s much more long-winded for the end user. The Make Rot From X also uses Quaternion calculations to make the rotator, so you don’t get a resulting rotator that has an unusable value. Like ranging from 180 to -180 for example (to my knowledge anyway).
If you want to use Timelines with Rotators (and there’s no current way), you can simply treat X, Y and Z as your pitch roll and yaw. Functions that produce vectors aren’t at all useful for rotators (for example, there’s no useful translation of ‘Get World Location’ to ‘Make Rot’). However, there’s nothing stopping you storing a rotators values as an FVector, just be aware that it’s not an FVector, it’s an FRotator stored in an FVector container. In fact, I bet at a much lower level in engine, the FRotator is no different to the FVector, but the operations performed on it are different.
The reason the ‘Make Rot From X’ exists is for an easy conversion from a vector to a rotator. If you want an arrow for example that points in a vectors direction, user Make Rot From X on the direction and apply that rotation (in world space) to the arrow.