[syg]

The ability to do unordered operations on shared memory is important in general to write performant multithreaded code. On x86, which is very close to sequentially consistent by default (it has something called TSO, not SC), there is less of a delta. But the world seems to be moving towards architectures with weaker memory models, in particular ARM, where the performance difference between ordinary operations and sequentially consistent operations is much larger.

For example, if you're protecting the internal state of some data structure with a mutex, the mutex lock and unlock operations are what ensures ordering and visibility of your memory writes. In the critical section, you don't need to do atomic, sequentially consistent accesses. Doing so has no additional safety and only introduces performance overhead, which can be significant on certain architectures.