many use it cuz its cheap. It was one of the first salts that i used, but have since ditched it and wont use it again now that i've had reef crystals, which is also cheap but much better.
oceanic can be very high in calcium...not sure it hurts anything, but its sure not needed. I was having it in the +500 range. I also had low pH with oceanic. My pH was always at 7.7. I did a couple little changes and it brought it up to 8.0, but when i switched to IO and reef crystals later on, my pH has been 8.3 ever since and i didnt do any more changes besides the salts.