The first reason being that it will enable quicker posting (by eliminating the "select your character" step on the posting page if the character you want to post with is your active one, as it will already be selected). In addition, you will have the option to have your active character switched upon posting with a different one.
With `Quick-switching` enabled:
You had "Jobe" set as your active character because you had only been posting with him for several sessions, but you decide to use your character "Saitia" for awhile. You simply change the character in the drop-down box by your post, and after you post, "Saitia" will now be your active character.
With `Quick-switching` disabled:
You know that your primary character is "Jaron", so you disable quick-switching. You decide you want to make a single post as another character, so you change the name in the drop-down box next by your post and submit. Afterwards, "Jaron" is still your active character.
The other reason for this addition (aside from the increase in ease of use), is that it will determine what languages can be read while browsing. Basically this means that if your active character can only read commonspeak, you will not be able to decipher non-commonspeak languages as you go along. We also don't want people to simply switch characters just to read another language. While I understand this will not prevent such (nor is there a way), it is a hinderance. Sort of.