Uniforms for porters are primarily used for easy identification and behavior regulation, similar to how military uniforms were employed to distinguish servicemen.
The most likely reason for having special uniforms for porters is to identify them as employees and to regulate their behavior. Special uniforms serve as a visual cue that distinguishes employees from customers, much like soldiers on leave were required to wear uniforms for easy identification by military police. In the context of a grocery store, a man in a suit is not typically assumed to be an employee because our understanding of social dress codes informs us that employees wear uniforms that are practical for their work environment.