It's not just tiny platforms. For example, you often want allocators with some predictable characteristic. Eg; constant time allocation for a given request size

It's not just tiny platforms. For example, you often want allocators with some predictable characteristic. Eg; constant time allocation for a given request size. Or you might want an allocator that keeps certain items contiguous so they'll tend to already be in cache. Custom allocators tend to be used when you have a particular requirement that a general purpose allocator isn't optimal for. For tiny platforms often it's bare-metal so there's only one running "process" and so something like "malloc" that needs to be aware of memory usage across the system isn't relevant. Moreover, embedded systems tend to care about deterministic performance so an allocator specific to the application can be a better match.