What's the difference between Field History Tracking, which needs to be activated and works only for 20 fields per object and History tables like AccountHistory, CustomObject__History ? Why would I enable Field History Tracking, if I can easily use those tables?
Converting comments into answer:
CustomObject__Historytable starts tracking the history of a field
customField__conly if you enable field history tracking for that field otherwise it won't.
CustomObject__Historytable is created only if we enable field history tracking for at least one field of that object otherwise the table won't exist.