Restoring from an LDF file
My question is, is it possible to just "rollback" the db to a few hours ago , using this huge LDF file 


No, You are in simple recovery, once the transaction is committed it is not kept in the .ldf there are no point in time recovery options. 


  and if not, what possible use does this LDF file serve if I can't restore from it?


You do not differentiate between the allocated and used space on .ldf file.  My best guess is that long ago it was in full recovery, and no one was doing t-log backups so it grew huge.  Then is was changed to simple recovery and it became mostly empty.  See related Why Does the Transaction Log Keep Growing or Run Out of Space?