This is to show the end of an action and imply the speakers feelings after. I think this expression is better used in spoken Korean. Intonations while speaking would help determine the tone and reactions a bit easier.
Examples of implied happiness:
다 끝내 버렸어요! = I finished it all! (and I’m so happy about that!)
고민하다가, 사 버렸어요! = I was hesitating, but I finally bought it! (and I’m happy I did.)
Examples of implied sadness:
벌써 다 말해 버렸어요. = I’ve already told them everything. (it’s done, it’s over.)
컴퓨터가 멈춰 버렸어요. = The computer has stopped. (it’s really inconvenient.)
**examples from TTMIK**