Deleting elements can be useful for when you have decided that a piece of content doesn't fit in your publication, you can then simply delete it and decide for yourself what you're going to do with that space.
Copying elements can be useful in order to keep a good workflow/speed and to allow yourself to create multiple elements without having to change all the settings for that element, you can copy an element and only change a few settings to fit that new copied element.
To delete or copy an element, simply select an element in your publication you would like to copy or delete. An icon will appear typically on the left side of the selected element, that icon is blue with a white cube inside of it.
Mouse over this icon and a black bar shall appear with various different options, on this bar you will see a Duplicate and Delete Icon. Clicking the duplicate icon will then create a copy of that element and place it above or below the original element. Clicking the delete icon will simply remove the element from the publication.