The wizard robe skirt (t) (previously the blue skirt (t)) is a level 1 treasure trail reward. It's the trimmed version of the normal Wizard robe skirt, and part of the full wizard (t) set. It is often worn for its visual appeal, as well as the fact that it can be used as a status symbol for free players that cannot wear and/or afford the slightly more expensive decorative rune platelegs (guthix platelegs, rune platelegs (g), etc). Due to it having no requirements, it is a good option for free to play skillers or defence pures who want to show off their wealth.
Free-to-play mages often wear this skirt in replacement of the wizard robe skirt to show wealth. This, the more expensive blue skirt (g), and the wizard robe skirt are the main options for free-to-play mages, as part of a set with the blue wizard hat and wizard robe top. All other free-to-play mage robes for the leg slot are rewards from minigames and might be harder to require since they cannot be bought.