The computer should give a short variation if it finds a move that's better than the one played. How often it does this depends on what your evaluation threshold is. eg. if your threshold is set to 0.3, the computer will only suggest a better move if it's evaluation is better than the original move by 0.3
To understand why White is better you have to use your own skill in analyzing a position, and if that's not up to the task, the help of a coach or stronger player. A computer will tell you what it thinks is objectively the best move, but that doesn't guarantee it is the best move for a human player...computers have no fear and will enter razor-sharp complications or difficult positions without second thought if it evaluates the resulting position as even minutely better
I've had Houdini analyze some of my recent games.
While it tells per each move made which side it thinks is doing better, e.g. +0.38 if White is doing a little better, it doesn't tell me what it thinks is the strongest move for the side to move.
Nor does it tell me *why* White is slightly better.
For both of these functions - what is the best next move and why one side is faring better than the other isn't that what one would need a human chess coach for? Just pondering. Can one really learn much of value from machine analysis?