Point of entering Treasure cave is to collect some rewards based on level of fights you have won for different portals.
Note that you can only enter treasure cave ONCE during whole event so, it is better done after you have reached a point where you cannot win any more portal fights to maximize benefit. But let me remind you that overall rewards of Treasure cave are very average.
Best reward of this event is Packs (can actually unlock a girl card for you if you are lucky) and Libido crystals (to upgrade Mythic girl ability).
TIP for newbie: Your first 5 libido crystals should be reserved for Mythic girl with direct damage ability upgrade (Yakuza/ Venus/ Black Mage/ Jungle Queen/ Desert guard) - But you need Mythic girl card to be level 5 for ability upgrade; you need 50 cards to level Mythic girl from level 1 to 5.