See thread: https://www.duolingo.com/comment/14057649
Although, not sure if that helps much. But it's the same structure as rhaid i fi; in the same way that it literally means "[there is] necessity to (me, you, etc.)" instead of just using a modal verb like "must," I guess this one would be something like "[there is] since to (me, you, etc.)" instead of using a conjunction.
I think it's probably included because it's the same pattern in Welsh:
something + i + person + mutated verbnoun
Rhaid i fi fynd "I must go"
Ers i fi fynd "Since I went"
Cyn i fi fynd "Before I go/went"
Ar ôl i fi fynd "After I go/went"
Gwell i fi fynd "I'd better go"
Nes i fi fynd "Until I go/went"
Erbyn i fi fynd "By the time I go/went"