The ZenFone 3 Zoom has one wide-angle lens camera with 25mm equivalent focal length and a second camera with a 59mm equivalent focal length. A 59mm focal length is equivalent to approximately 40.3° FOV, compraed to a much wider 81.7° of that of the 25mm camera. This is one of the biggest strength of the Zenfone 3 zoom rear-facing camera, because it serves like a zoom lens, enabling photographers to 'pull' the subject closer without moving, but it's main benefits it for shooting portraits.
Due to the lens design, portraits will appear with no distortions, compared to when shot with a wide angle lens up close. The higher focal length produces more prominent shallow depth of field effect, although to get the most noticeable blurry background effect, you want to use the Portrait mode which can produce a really stunning buttery smooth bokeh effect, perfect to diminishing and blurring out distracting background elements that can distract the viewer from the main subject.
Also a higher focal length also compresses the background, making background elements like houses and trees in the photo, appear like they are closer than they really are. This is another reason why it's better to not shoot portraits with a wide angle lens. There will be less background elements and even the blur effect will look better.
Usually in single-lens cameras, you have a single prime lens with a single focal length and this focal length is fixed, it cannot be changed. So every time you capture a shot, you need to compose the shot while understanding that you are being restricted to that particular field of view. Now with the Asus Zenfone 3 Zoom, you have more room for creativity.
Here is an informative videos by Adorama that talks about how focal length affects the background in portrait photography. The guy in the video uses a DSLR camera, but the same rule apply to mobile phone cameras, no just interchangeable lenses for SLR cameras.
So having a second camera with a large focal length is a big advantage, especially for shooting portraits, but for times where you just want to have the subject appear larger in the frame without physically getting closer to it.