Firstly, anyone who does mallocs a block, and then immediately does a memset(block, 0, size) on that block almost certainly should be using calloc. The Linux kernel has kzalloc for this purpose as an alternative to kmalloc and it's fairly widely used.

Sometimes it is inefficient to set something to zero, when that is not the true initial value (some or all of the zeros will be overwritten with the true initial value before the object is used). If you malloc some structure and then painstakingly initialize every member, then calloc is wasteful.

If you malloc some structure, and then forget to initialize some of the members, calloc will hide that bug by initializing them to all zero bits. This might not be the correct value. If the uninitialized parts of the structure aren't clobbered from zeros, you can get an accurate diagnostic about that from a tool like Valgrind. On the other hand, if you aren't using that tool, the behavior with zeros is more deterministic. E.g. if a pointer is not initialized, but overwritten with zeros, on most mainstream platforms that will be a null pointer whose dereferencing uses are detected. If you don't have such a tool available, having the uninitialized bits be all zero is arguably better.

Much of this reasoning mirrors reasoning about whether to initialize a local variable to zero, if its true initialization actually happens later.