This by no means represents the official reason, it's simply
my own thoughts on the reason behind the <code> tag.
It looks as if the tag is handled in a special way (just as
the square bracket links are). The fact that you can select
how code is displayed in user settings seems to indicate this.
Currently it looks as if the <code> tag is treated as
a <pre> tag.
One advantage of having the tag interpreted is that it's
easy to change what it does either globally or by user.
I imagine the term 'code' was chosen because the content
is, in fact, code, so it ought to be easy to remember.
Unfortunately it appears to be confusing as well. Using
<pre> would also be confusing, because there's no
guarantee <code> always is or will be directly
converted to <pre> (<code> seems to be
replaced with <pre><it> for me; if I turn
off 'large code font' in my user settings, it replaces
it with <pre><it><font size=-1>).