I experienced depression as symptom of a deeper "illness" of the brain that needs to be cured with psychotherapy. If it wasn't for that I'd still make the same mistakes again and again that made me depressed in the first place.
Depression is usually a cycle, I'm too tired to go to class, ends up failing classes, great now I'm a useless human, fails more classes, becomes too tired to do anything I normally enjoy, can't focus on homework, becomes more depressed, socializing causes too much anxiety to make friends, etc. This was me last year.
The hope is that with the correct combination of medications you'll manage to "break" this loop. You can start to focus again, maybe complete some homework. You have enough energy to get yourself to class. Your grades improve, you don't self-loath as much, your life improves, you make supportive friends.
Once you have your life back in your control you can consider discontinuing your medication. You might go off it and be fine, now that you were able to focus on your stresses in life and how to handle them, you may find you don't have any issues without your medication. Or everything becomes worse again and you have to stay on it.
To be honest, modern psychiatry is kinda a crapshoot and nobody knows 100% for sure how antidepressants work, why some people don't respond at all to prozac but do to cymbalta, etc. There's a lot of research involving your diet, your gut bacteria's health, your immune system's effect on mood, etc. All of these will be a factor.
The problem is when the sadness becomes the crippling kind that prevents you from acting to change the situation that's making you sad to begin with. It can be helpful, I think, to take anti-depressants long enough for you to be able to enact positive change in your life.
My wife was taking anti-depressants as she finished up college, then she went traveling, met me while we were both backpacking, we got married, have two kids. She weaned off the anti-depressants while we were engaged. She gets transitory anxiety attacks every once in a while, but she's told me she's never had a relapse of depression. I think she was just bored and lonely and stressed in college -- a temporary situation.
This is all just my personal opinion, i'm not a psychiatrist.
Depression is sort of different than generic sadness as it DOES NOT drive you to improve your situation, due to the severe hopelessness it causes. It's the massive problem with depression, and why medication can be so helpful. Depression can cause you to stop functioning, even at your situationally reduced level, essentially fucking you over even worse.
As you show, it was an external event that kicked your wife out of her rut. Sometimes that required kick is helped by medication
Meanwhile I expect to be on my anti-anxiety medication for my life, as my brain chemistry is not going to fix itself.
Think of it like Ibuprofen. It's there to help you manage pain, but it won't fix things.
Major Depressive Disorder is a specific disease that's commonly called "depression", and I have no idea what the long-term treatments look like there.
It can "numb" the feelings, like pain meds (as others have said).