There is no reason for you not to use any size that you want,
however, over the years photographers and designers have developed a group of "standard" proportions that they feel are appropriate to showing images in the best way allowing control of, and help with, composition ................ obeying the basis rules.
Such sizes are incorporated in most photographic processing applications and are used by printing houses when producing prints.
IMHO it is good to know and use these as a aid and a basis for any crops that you do........ they do matter as they will help you with your composition .. especially when beginning