This isn't about restoring a JPEG file back into its original RAW format. The information lost from converting RAW to JPEG is gone. There is no way to get that back.
This is about storing JPEG files more efficiently. DropBox is in the business of providing cloud storage, and it is in their best interest to keep their costs as low as possible. The more they can compress data for their customers, the more efficiently they use their infrastructure. Some files such as text documents are easy to compress. Some files such as JPEG files are difficult to compress, especially with lossless algorithms.
For DropBox, this allows them to store the LEP representation of a JPEG file instead of the actual JPEG file. This saves them approximately 22% of their storage needs. They can then decompress it on the fly whenever a user tries to read the original JPEG file, essentially trading savings in storage costs for a bit of extra CPU demand. As long as the compression is lossless and the user sees acceptable performance, there is no user impact.
Depending on the cost of extra CPU cycles vs. the cost of reduced storage, and the relative mix of JPEG files vs. other data files, this could save DropBox quite a bit of money.