Where we mostly use label in html

Answer 1

<label> is mostly used with the <input> tag to denote or label an input field the kind of input is expected.
Another alternative is to use placeholder attribute along with the input tag.

Answer 2

as regular input can have a placeholder
the label comes in handy when the input type is a checkbox or select tag or just angthing without placeholder actually xD

Answer 3

it is commonly used with input tag