For the most part it's not useful, at least not the way people use it most of the time.
It's an engine for producing text that's most like the text it's seen before, or for telling you what text it's seen before is most like the text you just gave it.

When it comes to having a conversation, it can passibly engage in small talk, or present itself as having just skimmed the Wikipedia article on some topic.
This is kinda nifty and I've actually recently found it useful for giving me literally any insignificant mental stimulation to keep me awake while feeding a baby in the middle of the night.

Using it to replace thinking or interaction gives you a substandard result.
Using it as a language interface to something else can give better results.

I've seen it used as an interface to a set of data collection interfaces, where all it needed to know how to do was tell the user what things they could ask about, and then convert their responses into inputs for the API, and show them the resulting chart. Since it wasn't doing anything to actually interpret the data, it never came across as "wrong".