Keyspaces group column families together. Typically, there will be one Keyspace for each application that uses a Cassandra cluster. The most important settings that are defined at the keyspace level are the replication factor and the replication strategy. Thus, if you have sets of data that have different requirements for these settings (such as different levels of fault-tolerance), these sets of data should receive different keyspaces.
A increase in the number of keyspaces does not inherently have a performance impact on Cassandra (although the number of column families does). With that said, keyspaces are not designed to be used as a significant “map” layer in the data model, only as a way to control data replication for a set of column families.