script tags are so you can post exactly that programming code.
Forums use BB code which strips down any submitted text of characters that are not allowed this is for two main reasons
1) so that users cannot change the way the board looks, if html characters were allowed then it would be easy to deface the board.
2) for security reasons lots of exploits are openend up if certain characters are allowed.
So the script tags allow for code to be posted without it effecting the board or opening up exploits
PRE I presume comes from the html PRE tag which is preformated text
to quote an HTML source
The PRE element contains preformatted text. Visual browsers should render preformatted text in a fixed-pitch font, should not collapse whitespace, and should not wrap long lines.
PRE is useful for formatting computer code or poetry where whitespace is important, but since preformatted text is inherently visual, authors should avoid dependence on it wherever possible.
Also it shouldnt be parsed by smilies which I have just noticed here that it is
http://minibbtest.virtualis.lv/minibb-test.php?action=vthread&forum=2& topic=96
But that is an example of code posted on the link above
