text: Q:
  
  Seemingly unnecessary verbs in comparisons
  
  I've come across a strange habit in comparisons that seems to be pretty popular. Instead of saying (what I think to be correct):
  
  He runs faster than Robert.
  
  Sometimes I hear or read:
  
  He runs faster than Robert does.
  
  What is this? Is it a "colloquial but not perfectly correct" form like "try and do" instead of "try to do"? It feels so unnecessary and redundant to me.
  
  A:
  
  As @P.E.Dant writes, dropping it is ellipsis. So if either option is less careful, it would be dropping, not adding, the second verb. (That said, both are very common in every register of speech.)
  The "redundant" option is often used to avoid ambiguity, because the other side of the comparison could be the subject or the object of a previous verb.
  Strong Sad of Homestar Runner says at one point:
  
  I like board games more than most people.
    — Strong Sad's Character Video (wiki page with transcript) (video)
  
  Interpretation (1): Does "most people" match "I"?
  
  That is, do both Strong Sad and most people have an opinion of board games?
  
  Interpretation (2): Or does it match "board games"?
  
  That is, does Strong Sad have an opinion of both board games and most people?
  
  Hmm... !
  Strong Sad goes on to say that both of those are actually true in his case:
  
  (1) By that I mean I like to play board games more than most people do.
    (2) But by that I also mean I like board games more than I like most people.
  
  In clarification (1), he repeats "like" by adding "do", which would be redundant except that it forces "most people" to become the subject of the verb. (The added "to play" doesn't affect this.)
  
  So both Strong Sad and most people have an opinion of board games.
  
  In clarification (2), he repeats "I like" before "most people", which would again be redundant except that it forces "most people" to become the object of the verb.
  
  So Strong Sad has an opinion of both board games and most people.
  
  But usually, you only want to say one of the two options in such a comparison.
  So you can use those redundant verbs to resolve that ambiguity, which comes up fairly often in everyday life. Most of the time the context makes the speaker's meaning clear, but not always.
  And sometimes the sentence doesn't really present ambiguity at the semantic level, as in your example. "Robert" can't be the object because he's not a measurement of speed (compare "He runs faster than 50 km/h"), so Robert must be the subject of an implicit, elliptical second "runs". But the syntax still makes him sound like an object, so we reinforce his subject role by adding the verb. That's my theory, anyhow.
  
  Edit: @Muzer makes the point that some pronouns in English are cased, e.g. "I" (subject) vs. "me" (object). In theory that's enough to distinguish the two interpretations for those pronouns. That said, s/he also mentions prescriptivist grammars and I think there's something to that. The average speaker uses either "than me" for both meanings, or "than me" vs. "than I do". Ending the sentence at "than I", economical though it may be, can sound somewhat pretentious!
  
  P.S. Don't attach too much importance to anything Strong Sad says in that video. :) That said, Homestar Runner can be a goldmine for linguistic curios if you're nearing fluency. They often play with very subtly odd uses of English — or downright outrageous ones, which can be hilarious.
  
  A:
  
  The first sentence is an ellipsis. The word ellipsis (plural ellipses) means "omission". Ellipses are common in comparative sentences like these. 
  The first sentence without the ellipsis is:
  
  He runs faster than Robert runs.
  
  In this case, the ellipsis is a repetitive reduction of the verb run. The verb  "belongs" with both sides of the comparison, and is elided to avoid repetition. Native speakers use this form of ellipsis frequently, as in:
  
  She sings better than Bob sings.
  
  The second sentence is an example of replacement of a repeated verb with the auxiliary verb "Do". Rather than repeat the verb runs, the speaker replaces it with does (which is understood as does run.)
  
  He runs faster than Robert runs does.
  
  This is also common in comparative sentences, such as:
  
  She eats more than Bob eats does.
  
  The value of the shortening afforded to both speaker and listener by this common replacement is less obvious in these examples than in a sentence like:
  
  The Volpars of the planet Z'arx redistributed more wealth than their rivals redistributed did.
  
  Both of your example sentences are perfectly correct and idiomatic English. See also this question.
  
  