You don't clear or discard your hand immediately after destroying a monster. The cards you used stay just because you might get a raid effect that makes you destroy a card. Only after the dungeon is filled and any raid effects carry out do you then discard your hand and end your turn.
So its actually a good effect since player can use it destroy unwanted card after battle?
Sometimes it is!
Although keep in mind a rule I always forget, that you cannot destroy a disease to satisfy a monster's "destroy one card" effect, unless that effect explicitly says to destroy a disease. You could still destroy other trash from your hand.
Also, Raid effects are triggered whenever a monster with a Raid effect enters the dungeon, which is usually after a battle, but can also occur before the battle (e.g., if an ability banishes a monster from the hall, and then refills the hall).