The aim of rerolling is to obtain an edge in competition at the start of the game by obtaining high performance weapons or to obtain the desired character/job class that you want. Rolling gacha in SinoAlice is the very first thing you do when you start off SinoAlice.
Note: These are the starting characters with alternate jobs that are obtainable through gacha. They are NOT the actual character jobs obtained through the story mode.
There are effectively NO tiers on characters. You personally become stronger over the process of limit breaking weapons, character rank up and the release of higher cost weapons. Rerolling is overall most effective for min/maxing your stats since characters all possess the same base stats if you level their job levels up to 10.
iOS requires you to delete the game and then re-download from the app store.
To delete the game following a roll involving characters you don't like, Clear app data.
Settings > App > SINoALICE > Storage > Clear Data
Once you are satisfied with your reroll result, click on the menu button at the very bottom of the screen (indicated by a Gear icon) and click on the button with the circle around it. This will backup your data so that you don't lose your account even if your phone were to have an accident! Just follow the instructions on screen and you should be set.