HTML is not about presentation. It is a way of describing data. If you have some text that represents a label for an input, you wrap it in label tags not for presentation but because that’s what it is. Without the label tag, that text is almost meaningless. With the label tag and its for attribute (or not*) you are providing meaning and structure and forming a relationship between your markup that can be better understood by computers/parsers/browsers/people.
* you don’t necessarily need the for if you wrap the label around the input:
<label>My input
<input type="text" id="my-input" />
</label>