[SDPhantom]

Most of the "protections" are to prevent people from writing "bot" addons. There are also some blocks they added to prevent people from running malicious code. Malicious code from an addon isn't capable of causing harm to your computer, but historically was capable of selling/destroying all of your items or manipulating mail and trading to give stuff to another player.

While ShowUIPanel() by itself seems innocuous, it has to deal with shuffling windows around and closing ones that don't "fit" on the screen. Many of them like the Spellbook are protected and as such, requires ShowUIPanel() to be run from secure code.