I wouldn't call it "manual memory management", but "explicit memory management" (e.g. to have complete control when and where memory allocation/freeing calls happen, and that memory management is "obviously visible" in the code).

Explicit memory management is very important when performance matters and must be predictable (for instance in "soft realtime" applications like games), you want to get the expensive memory management calls out of the hot paths as much as possible and the language must provide the tools to make this possible. When seen under this lense (that memory management should be explicit and not hidden from the programmer), there isn't all that much difference between C, Zig, C++ or Rust, all those languages provide the features for explicit memory management, even if some also provide optional features for automatic memory management.