Dark Realm is the only where you can restock your essential items specially Herb and Medicine. In average, you spend 10 to 15 minutes to complete the Dark Realm. The items locations are different depending on where you enter the Dark Realm, either from the cocoon man in the sentry room or in the well. Here is a list of prizes except Fluorite and Bishamon Ocarina where you can pick up them only once, they will disappear when the second you enter the Dark Realm.

Limited items:

• Fluorite (x3)
• Bishamon Ocarina (floor 20th)

Unlimited items:

• Medicine
• Burst Bullet/Soul Absorber
• Herb (x3)
• Talisman

The are a few reasons you want to enter Dark Realm multiple times:

• You are a beginner and want to collect more Herbs/Medicine for assurance.
• Getting S Rank requires you kill more than 600 monsters that is why the Dark Realm is the best place to grind. 
• Maximize Weapons and Orbs for achievements.