Why is it useful to create a personalized builder if on it is also included the platforms supported by the default one?

[gmartinez1985]

There is something I'm missing about the usefulnes of creating a builder.

In my case I'm using a Mac device with arm64 architecture. I have Docker installed through Docker Desktop. My Docker version is 27.0.2, build 912c1ddf8a.

By default there are 2 builders, supporting both of them the same platforms:

If I create a new custom builder por the architectures riscv64 and 386 (and also an invented one), when I run it is added to the list of my 3 platforms all the supported platforms by my Docker installation.

In fact, if having the following basic Dockerfile:
FROM hello-world:linux

I execute the build for riscv64 and 386 (the 2 existing arquitectures I included in my builder), and also for amd64:

3 executions are succeed when in my custom builder I didn't include amd64

So, why is it useful to create custom builders if in any of them it is also to be included all the supported platforms by the default one?

Greeting and thanks in advance.