According to my gynecologist at the time (and this is all just from what I remember being told; I'm not a medical professional by any stretch), it was originally developed to help treat uterine diseases & infections, so it tells your body to stop producing the uterine lining at all (rather than telling the body to just not slough it off until a prescribed time, as some bc methods do – the patch, for example). It takes several injections to fully take effect (in terms of menstruation, I mean; I'm sure the actual bc aspect works faster). Some people stop menstruating entirely, while others will still 'spot,' as was my experience, tho after the first few months it was never anything more than what an underwear liner (not even a full pad, I mean) could handle, and rarely lasted more than a day. Granted, even that much may still cause dysphoria, but for myself, anyway, it made a huge difference.
It does take longer to leave your system than other bc methods, iirc, something like 6-12 months from the last dose before you have any reasonable possibility of pregnancy. So it doesn't permanently prevent you from getting pregnant if you want to in the future, but it can mean waiting longer from the time you decide to start trying until you succeed.