ᐅ What are the advantages of 24mm perimeter insulation?

Do you want to leave it out or what? I think the picture clearly shows why uninsulated corners are problematic..

Yes, I want to leave them out.

But you can see that despite the insulation, there is no noticeable difference in temperature, especially because the masonry still feels very cold on the outside.

You can’t really judge that based on the picture. I assume you are building with solid construction without external thermal insulation composite systems (ETICS). However, the picture shows a wall assembly with ETICS. Omitting insulation under the slab already has the potential to cause problems.

The person responsible for calculating the energy performance certificate (EPC) will be able to answer your specific question. Only that person knows the exact conditions and can assess the entire system.