Why would you use the <label> attribute as opposed to something like <h1>? Are there any main differences/reasons?

Semantic markup encourages us to use tags/elements that describe the meaning of the content. In a form, the word(s) outside the INPUT (data entry box) are more clearly described as a LABEL than they are a heading like an H1 or H2, or a generic piece of text like a P.

In addition, there is the helpful behavior you mentioned, which is really quite nice. When associated with an INPUT, a user "click" on the words in the LABEL will move their insertion point inside the data entry box. An H1 or P won't do that.

I think the main reason for using a label tag is semantic markup. Whereas h1 is meant to indicate the weight of a piece of content in the overall doc outline, labels indicate that a piece of content is a label for a form element on the page.

If that's the argument, using a label in this case is for semantic's sake, why not use another fieldgroup/legend combination? After all, you are defining a group of fields (checkmark or radio inputs) for one particular value that could then be described by the fieldgroup's legend.

But I realize there could be many solutions to this pattern.