It's common use is as the second chunnin exam, though chunnin and jonin level shinobi are aloud to use it as a training ground, Gennin however are forbidden admittance with out a jonin sensei to guide and watch after them. Though that doesn't stop all of them. Inside there are all manner of creatures, as well as toxic plants and other hazards, there are even left over traps from previous chunnin exams. Only a fool would enter and train by themselves, or someone extremely confident in their survival skills.