It is just "insider lingo." Because the airlines use single letter computer codes to identify different classes, it makes knowledgeable customers feel like insiders to use them as well. It can also be argued that using the correct codes represents precision over using general terms.

You might be interested to know that there are not simply one first class, one business class and one economy class codes. There are codes for fully refundable seats in each class, different codes for partially refundable, non-refundable, deeply discounted and so on. That is where all the different codes come from.