There are various bits and pieces one'll have to consider when taking a photo. You'll have to consider the light, appropriate lens and filter, composition, angle, depth of field, aperture, shutter speed, ISO, flash and various other elements. Likewise, the orientation; or the mode of the photo is also another important choice one has to make and that makes a lot of difference.
The main idea is to fit the subject into frame, however some kinds of photos are just suited for landscape mode while others are best when taken on a portrait mode. Most people choose one or other depending on what they would like to have in their frame. One'll have to change between the modes to fit the subject they want to have in the photo. Basically, portrait mode is used to take photos of the subjects that are vertical while landscape mode is used to take photos of subjects that are vertical.
A tall building or a person will fit naturally within the vertical frame. The vertical framing will also give the impression of elongation and it'll correspond with our natural visual experience. Therefore portrait mode is the first choice for most photographers to take vertical photos. Similarly, landscape modes are preferred for horizontal subjects to match with our natural visual experience. However, photography is not just a science but also an art. Sometimes breaking the rule is more relevant! Feel free to try different orientation and feel the difference...
However, if you are recording a video or taking a photo for video, never go for the portrait mode. I know you can understand why!
