Did you take a look at snapshots? Each keyspace has its own "snapshots" directory. You can delete them. Or (if on RHQ 4.13) set up Snapshot management in Administration -> Topology -> Storage Nodes -> Cluster Settings. Snapshots used to be created within the storage maintenance job every week prior RHQ 4.13, in 4.13 it has been disabled by default. I think snapshots are also created at storage cluster topology change (node leaves, joins, and maybe other events).
so what does the snapshots are used for? Can I delete them completely or shall I keep them for some time like a week or so?
RHQ or cassandra does not use(read) snapshots at all. It's a backup.