You don't "use" a "soft return," which is Word's natural word wrap at the
end of a line (which can be controlled somewhat with margin settings,
indents, and nonbreaking characters but otherwise not forced).
If you're referring to a line break (Shift+Enter), you use it any time you
want a new line but not a new paragraph. A good example of the use of this
is in typing poetry. The paragraph style used for the stanza can include
some Space Before/After (to allow extra space between stanzas), but the
stanza itself is single-spaced, so you insert a line break at the end of
each line (verse) and a paragraph break at the end of each stanza. Keeping
the stanza in a single paragraph also allows you to format the style, if
desired, as "Keep lines together" so it will stay together on one page.