What if I set width for block element?

Answer 1

Oh! I knew it although you set width for block element. It doesn't let other elements to be in that line. So mean ...lol..

Answer 2

even if you set the width that element will keep the entire line space, so like Namas said, another element will be placed in the nex line of the page.
But you can use custom width if you plan to draw a border or define a background