Both of the Samsung Galaxy A7 (2017) cameras, there rear and the front, feature a fast f/1.9 aperture lenses. We usually get to have a fast aperture lens only for the main camera at the back of the phone and an inferior setup at the front. This is not the case here, and for you this means that both cameras cap perform admirably in low-light situations.
In the image below you can see a side by side comparison that illustrates the relative size difference between various aperture f-numbers.
You can see that the larger the f-number, the smaller the aperture size is. So although it might be confusing at first because you expect the smallest f-number to be the smallest, it just works the other way around. so when you see a mobile phone cameras with a f/1.9 aperture, this lens will collect more light compared to a f/2.0 and a f/2.2 aperture lenses, but less light compared to a f/1.7 or f/1.8 lens. For example, the LGV10 and LG G4 come with a f/1.8 lens, the Samsung Galaxy S7 has an f/1.7 aperture lens and the LG G5 and iPhone 7 has a f/1.8 lens. The iPhone 7 Plus has two cameras, one is a wide angle lens with a F1.8 aperture lens and the second is a telephoto lens with a slower F2.8 aperture lens.
So although the Samsung Galaxy A7 2017 rear and front facing cameras aren't the fastest one available, there are still excellent for low-light photography, and you get that fast aperture for both cameras.
So why it's good to have a fast aperture lens for the front cameras? In my opinion this is for two main reasons. The first one is quite obvious, which is to be able to shoot at night and in situations where there isn't a lot of ambient light to get a well exposed image. Second, a faster aperture can produce a shallower depth of field (background blur) effect, and this is quite useful for self portrait shots where you shoot the image pretty close to your face (the closer you get to the subject, the smaller the depth of field will be considering same aperture and focal length) and it's nice to see the background blurred a bit because it blurs distracting elements in the frame, not by too much, but a bit.
So overall, this is a great advantage in favor of the Galaxy A7 (2017) and that's why I tagged it under the Pros section in the camera category.