Adventuring in bonus regions allows you to receive substantially more EXP and Gil, as well as fusion-specific cactuar cards.
However, note that each incursion into a bonus region has a time limit. Once the time limit is reached, the region is once again locked to you, unless you use more Mystic Tablets to enter it once more.
Bonus region reset
Select the bonus region from the world map and tap Reset to attempt the region again.
Mystic tablets will be spent upon reattempting the region.