Asking the OS for memory is a low-level operation, which means it is not portable and should be done by a library or framework. Also, since it's a syscall (involving a context switch), you don't want to do in performance-sensitive code.

Some applications might benefit from allocating huge pages. By default, the page size is something of the order of 4KB or 16KB. Large numbers of pages result in huge overhead. By allocating fewer, large pages on the order of MBs can help.