Exactly what
@BrendaP says. A lot of phones and tablets don't have a 'right way up' - they flip to the way you hold them - and the forum software can't make a correct judgement unless the orientation has been set by the originating device.
Equally, the software receives the image file in the size it is sent, and although it then
displays it so that it will fit into a post (approx 900px wide), the entire image is stored on the server and downloaded to each user's device when they access the page. As many phones and cameras these days take images that are 4,000px+ wide this is a lot of extra data to store and download, and slows the forum down considerably.
That's why we ask people to change their settings so that the images are smaller to start with, edit them on their phones, or use lunapic (which, incidentally, also deals with the orientation issue).