Actually, there is. United started adding the letters to the end of the callsigns as a marketing/revenue-enhancing move. Itineraries with one flight number, even if they are two separate flights, show up at the top of the list of possible flights on booking engines. And people are most inclined to book flights that come up at the top. (They also save bit in the amount of miles they have to give out.)

So what happens is each segment of, say, OAK-DEN-BOS, for example, could have the same flight number, but the letter is added after one of the flights to help ATC minimize confusion. This was explained in a memo sent out several months ago to the FAA/ATC to explain what would be taking place.

Heavy means something completely different, as mentioned above. And yes, you won't hear it from centers where standard separation takes care of wake turbulence issues.