Critical is a multiplier chance to your damage. The reason why you stack critical is simple, would you like a slightly higher base damage, or a slightly higher chance multiplier. In practically all cases, the bigger chance multiplier will result in more damage.

Critical Hit: base chance at which you are likely to get a critical attack

Critical Rate: base chance plus any multipliers that add to it (for instance TenTen has a passive that gives a critical rate to your team)

Critical (in function stats): Nobody really knows because the Oasis devs (not a moderator) wont say how exactly it works. But having more of it will make it more likely for you to have a critical attack.