Think about reading a book, is the text or images in the center of the page, where is the page number located, and are the pages numbered consecutively? Typically, such things are pretty straightforward and follow conventions. Therefore, to some extent, we begin to expect things to be the same, especially if they are located in the same place, such as page numbers or even the next button. In the context of games, players also expect the same kinds of interactions, not only with gameplay but also with other on-screen elements, such as the UI. For example, if most games show health in a rectangular bar or with hearts, then that's something that players will be looking for when they want to know whether or not they are in danger.
The design of a UI needs to consider a number of things. For example, the limitations of the platform that you are designing...