It's a useful feature, and it could reduce some types of abuse, particularly if the original tweeter is, say, Bill Gates and the replies include those from scammy imitation accounts. The label, along with the blue verified checkmark, could make it more immediately obvious when Gates himself is replying, and not, for instance, @Bi11Gates.
Twitter confirmed the scope of the test to TechCrunch, with director of product management Sara Haider noting "we're exploring adding more context to discussions by highlighting relevant replies -– like those from the original Tweeter." Along these lines, a recent test included algorithmically sorted responses and nested, color-coded replies.
The tag is perhaps not the most elegant way to denote the first person to tweet in a thread, though -- some have described it as "ugly and distracting." Reddit, for instance, simply uses a microphone icon to identify a thread's original poster in the comments. Still, this latest test ties into CEO Jack Dorsey's aim to "increase the health of public conversation" on Twitter.