As the others have said, indivisible operations. This is important in the context of race conditions, imagine two threads incrementing a counter with no locks.

As the others have said, indivisible operations. This is important in the context of race conditions, imagine two threads incrementing a counter with no locks. Using non-atomic ops to read, increment and then store the number will lead to a bad time. Or more on topic, checking if a file exists and then trying to open it - lots of bad things can happen.