Why?
1)Add balance in your pictures
For example, in your shot where you attempted to put yor subject in in the dead centre or slightly around that.You could find your balance of shots by croping your images(You could use the rules of compositions) to fix it.
In this picture i use the rule of thirds and rule of space when im cropping
2)Remove distractions
Maybe sometimes when you are viewing your image,you find something that very distracting,you wish to remove it.You could use this 'cropping' technic to remove it.
3)Change to new formats
Cropping could also be used to change your image format.For example,if your origanal images is horizontal, you could change it to vertical if you want.You could also change your image to any non standard size if you want.But if you change you image to non standard size, you maybe will have problem to print your image because its not in standard size.
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