One difference between the CQL INSERT and UPDATE statements is in how they handle lightweight transactions. Lightweight transactions are essentially a way for Apache Cassandra to enforce a sequence of read-and-then-write operations to apply conditional writes.
Lightweight transactions are invokable at query time. Apache Cassandra implements the paxos (consensus algorithm) to enforce concurrent lightweight transactions on the same sets of data.
A lightweight transaction in flight will block other lightweight transactions, but will not stop normal reads and writes from querying or mutating the same data.
In any case, an INSERT statement can only check whether a row does not already exist for the specified the PRIMARY ...
