why amd64 and i386?

[martin30002]

I have one machine with ubuntu 20.04 where all packages are installed as both amd64 and i386. On another machine only amd64 packages are installed.
Can I get rid of the i386 packages?

[deadflowr]

all packages are installed as both amd64 and i386
They aren't. Only a handful ( a large handful) of packages have been built for 32-bit.

It means you have the system configured to use i386 packages, meaning something you use requires 32-bit packages.
(Wine is a common package set that still requires 32-bit, for example)

See here for a breakdown of what's what in regards to 32-bit on 20.04 as well as a list of 32-bit packages built for it:
https://discourse.ubuntu.com/t/community-process-for-32-bit-compatibility/12598