<human>: Give me a shortened version of the text

I have a gmail that hasn't been used in about 5 years (it was temporarily set up for a project). It was set up however to auto forward emails to my main email. Last week I received a load of messages about a facebook account being set up and realised it was coming from this email account. I logged in, changed the password, then logged into the new facebook account and deleted it. The facebook account was set up by an Indian man and he appeared to be uploading his own personal photos and adding his own friends (but under my email). 
2 questions 
- how would my gmail have been hacked when the password is NEVER used elsewhere or at all in the last few years (it also was an obscure phrase made up of letters and numbers so no guessable)
- But mainly what was the hacker up to? What benefits could have come out of setting up a personal facebook from a hacked gmail account? It really puzzles me and googling it can't find similar stories/explanations.
<bot>: how did a gmail that has a good password which never gets used get hacked, and what benefits/scam was happening when they used it to set up a personal facebook account?