These Bind to Account tabards are likely purchased at revered (Illustrious) and exalted (Renowned) reputations with your guild, making leveling additional characters through guild reputation that much easier. Additionally, assuming you get to keep the item upon leaving a guild, it should make the re-leveling process that much easier. Losing your exalted guild reputation for one reason or another and ending up neutral in your next guild can be a painful, painful thing. These tabards will help soothe that pain.
Judging by the sell-to-vendor prices on Wowhead, the Illustrious tabard will cost 125 gold and the Renowned tabard will cost you 250 gold.