This allows for the file to be uploaded to alwil, for analysis, and can also help you in the long run.
Say a file of a program you use frequently is identified as malware, but you are unsure you can allow it to be sent to the chest and then checked out further, rather then delete it and not know whether it was genuine or not
You can leave things in the chest for as long as you want but the usual time frame is 2-3 weeks, re-scan it and take actions depending on the result.
also the virus chest is a secure place where the files are unable to be accessed.
Hope this helps,