Useful is a common meme among Postfurries. The general usage in chat is to start a sentence with "x is..." and leave it hanging for other people to automatically complete with the word "useful." This response may be a hypnotic trigger for certain Postfurries, or it may just be force of habit... after a while, does it really matter which is which?
It may sound like a small strange thing, but for those involved, it's a combination of Postfurry guild countersign, reminder of being hypnotically trained, and occasional raucous group activity.
The original use of Useful as a term came in 2005 from the mind-controlling weasel character Corasyn on Puzzlebox (see alt User:Trouffee for more details). Useful at that time was defined as "the least intrusive method by which to control someone's mind." The mentally-inserted fact that an action or object was Useful meant that it was pleasant or preferable to do (without directly telling a person to perform said action or with said object).
The more modern usage came from a later introductory hypnotic session between User:Trouffee and Kincaid, in which the term repetition is useful was itself used several times to explain what a hypnotic trigger was and the best method to implant such a trigger. After that session, Kincaid could not help but finish the stated sentence "repetition is" with the callback "useful." The term later expanded to include any "x is useful" sentence and spread as a meme/hypnotic trigger among many Postfurries.