If I call some function foo() I have no idea if it does any heap allocation internally.

In zig I always know because all functions that allocate memory explicitly request an allocator. That also allows me to control which allocator they use.

It's more explicit to pass an allocator to things that will allocate than not to, in the same way that other local variables are more explicit than other global variables. By cultural convention, you'll be able to bring your own allocators to almost any library, so you won't have to let your libraries decide when your program performs heap allocations.

For many types of programs it's really, really important to have control over allocation behaviour which malloc doesn't provide (for instance a per-frame bump allocator, an allocator which reuses memory blocks but still works within a pre-allocated memory chunk, a specialized allocator for GPU memory blocks, etc...).

Zig makes it supremely easy to use different allocators for different pieces so you can do much better than just calling malloc everywhere. This enables very easy and straightforward use of arena allocators in particular.