The difference between a paragraph and a line that is alone

Not every block of text with space around it is a paragraph, and the distinction is not pedantry.

A paragraph is a unit of prose. It carries a complete thought and it is meant to be read as a whole. Marking text as a paragraph tells readers that the words inside belong together and that the boundary is meaningful, which allows somebody listening to move forward one thought at a time rather than one line at a time. That is a real navigation tool. It is also the default and correct home for the vast majority of the words on any page, which is worth saying plainly because beginners often reach for something more elaborate when nothing more elaborate is needed.

The trouble comes with short pieces of text that are not prose. A caption under a picture, a label above a figure, a piece of legal small print, a byline, a date, a single word in a corner. These are often wrapped in paragraphs by reflex, because a paragraph creates the spacing that looks right. Sometimes a paragraph is genuinely correct. Often the content has a truer description available: a caption belongs with its figure, a heading is a heading even when it is short, and a group of related fragments may really be a list or a set of pairs.

The other half of this problem is the opposite: prose that has been chopped into separate pieces to control where the lines break. A sentence split across three blocks so it falls neatly on three lines is not three thoughts. It reads as three thoughts to anybody who is listening, with a pause in the middle of a clause, and it falls apart entirely at a different text size or window width. Line breaks are a visual concern, they have visual answers, and they should never be achieved by breaking the sentence into pieces that claim to be independent.