why use modules if PCH on your diagram is not much worse in compile times?

Macro hygiene, static initialization ordering, control over symbol export (no more detail namespaces), slightly higher ceiling for compile-time and optimization performance.

If these aren't compelling, there's no real reason.

Having implemented PCH for C and C++, it is an uuugly hack, which is why D has modules instead.

modules are the future and the rules for are well thought out. Ever compiler has their own version of PCH and they all work different in annoying ways.