The "Force new revision" checkbox is a way to force it to create a separate revision each time you save.
The preference settingFORCENEWREVISIONCHECKBOX
controls whether this is checked by default or not. You can set this on site level, web level, topic level or user level.
On a related note, you can force every save to be a new revision number by setting ReplaceIfEditedAgainWithin to 0
.