Futures tickers use the month symbols. So a two year treasury future in CME might be "TUU5" where "TU" is a prefix symbol for any two year treasury future and U represents the delivery month, in this case the 3rd quarter month (September) and "5" is for 2015. I hope the benefit of such a succinct string for the contract is clear. In fact, once you know the product many traders will just refer to the month and year portion of the ticker, eg U5 instead of TUU5.
In practice even many futures traders only know by reflex the quarter month symbols (H M U Z), and some contracts only have quarter expires (commonly, nearby months may have contracts but as you go further out contracts may exist only for quarter months).
Why did they skip to F? Note there are other letters skipped including another range: RST. I've never been given an utterly convincing explanation but usually people think it has to do with picking letters that sound distinct, also when written on slips of paper in high time pressure (L and I can look the same and so are skipped). Perhaps BCDE all sounded too similar (imagine consecutive month contracts like D5 E5 or B5 being shouted to you in the pit).