The advantage of UUIDs over auto-incrementing integers is that you can generate them distributed. When using incrementing integers there must be a single counter somewhere that always have to be consulted when generating a new ID. With UUIDs you can just generate a new ID anywhere in your cluster and use it right away.
Basically you can think of UUIDs as big random numbers. So it's highly unlikely that two nodes are generating the same ID even if they are not coordinated.
Still it seems you should make yourself familar on the concepts of the keys in Cassandra. Different to relational databases, keys in Cassandra are not just there for generating a unique identification of a record but to prepare your query for data. Therefore keys in cassandra are often not a UUID … or not a UUID alone.