"It is ___ that/who + verb." pleonasm vs. "___ + verb."

Is there a place for using these pleonasms:

• "It is John who runs." (instead of: "John runs.")

• "It was congress that legislated." (instead of: "Congress legislated.")

?

It would seem to have its use only when what comes after the "that" is a long clause: Example from the intro. of Strunk & White:

It is an old observation that the best writers sometimes disregard the rules of rhetoric.

could be rewritten as:

That the best writers sometimes disregard the rules of rhetoric is an old observation.

But this isn't conciser.

In more formal grammatical frameworks, the construction you're describing is called an "it-cleft." The standard view is that it-clefts are licensed when the clefted constituent ("John" in "It was John who...") is a FOCUS. A FOCUS is a discourse function that expresses new, exhaustive, or contrastive information. In contrast, the it-cleft is not licensed or sounds odd if the clefted element is a TOPIC. A TOPIC is old information. To illustrate, consider the example below:

(1) a. Which student ate an apple?
b. It was [FOCUS John] who ate an apple.
(= John has not been mentioned in the question, new information, FOCUS, so it's acceptable)

(2) a. What did John do?
b. ??It was [TOPIC John] who ate an apple.
(= John has been mentioned in the question, old information, TOPIC, so this sounds odd)

There is much, much more to say about this, but this would be a good starting point.

Here is some literature on the topic.

Gundel, Jeanette K. 1974. The role of topic and comment in linguistic theory. Ph.D. dissertation, University of Texas at Austin. Re-issued 1988, in Jorge Hankamer, ed. Outstanding Dissertations in Linguistics, Garland Publishing Co.

Prince, Ellen F. 1978. 'A comparison of wh-clefts and it-clefts in discourse.' Language 54, 883-906.

Reinhart, Tanya. 1981. 'Pragmatics and linguistics. An analysis of sentence topics.' Philosophica 27, 53-94.

Rizzi, L. 1997. 'The Fine Structure of the Left Periphery. In L. Haegeman (ed.), Elements of Grammar. Handbook in Generative Syntax. Dordrecht: Kluwer, 281-337.

Rooth, Mats. 1985. Association with Focus. PhD thesis. Amherst: University of Massachusetts.

Emphasis is probably the usual driving force in such (and related) cleftings (as exemplified by your first two sentences):

From ThoughtCo: Richard Nordquist (bolding mine):

In English grammar, a cleft is a construction in which some element in a sentence is moved from its normal position into a separate clause to give it greater emphasis. A cleft is also known as a cleft sentence, a cleft construction, and a cleft clause.

"A cleft sentence is a sentence that is cleft (split) so as to put the focus on one part of it. The cleft sentence is introduced by it, which is followed by a verb phrase whose main verb is generally be. The focused part comes next, and then the rest of the sentence is introduced by a relative pronoun, relative determiner, or relative adverb. If we take the sentence Tom felt a sharp pain after lunch, two possible cleft sentences formed from it are It was Tom who felt a sharp pain after lunch and It was after lunch that Tom felt a sharp pain."

Take, for example, the simple declarative sentence, Jerry went to the movie yesterday. If you would want to emphasize one element or another, the sentence could be rewritten in several different ways:

• It was Jerry who went to the movie yesterday.
• It was to the movie that Jerry went yesterday.
• It was yesterday that Jerry went to the movie.

English has many different varieties of cleft constructions, but the two major types are it-clefts and wh-clefts. Wh- clefts use "wh" words, which is most often "what" in the construction. However, why, where, how, etc. are also possibilities.

That said, other perceived benefits are avoidance of awkward(-sounding) sentences:

• [Hmm] That the best writers sometimes disregard the rules of rhetoric is an old observation.
• It is an old observation that the best writers sometimes disregard the rules of rhetoric.

[Note that this similar-looking sentence is not a clefting; 'that' is the complementiser here, not a relativiser: Thanks to Richard Z for pointing this out.] Also, when the first gap in your template is filled by an abstraction such as an idea, a more formal, solemn, dignified sentence can be generated:

• It is an indisputable truth that the Holocaust took place.

[Again, complementiser 'that'.]

On the other hand,

• John snores

cannot be replaced by

• It is John who snores

in a simple statement of fact. The cleft sentence is marked for emphasis, identifying the guilty party.