From the Introduction [1]

    Many container platforms are available, but Apptainer is focused on:

    Verifiable reproducibility and security, using cryptographic signatures, an immutable container image format, and in-memory decryption.

    Integration over isolation by default. Easily make use of GPUs, high speed networks, parallel filesystems on a cluster or server by default.

    Mobility of compute. The single file SIF container format is easy to transport and share.

    A simple, effective security model. You are the same user inside a container as outside, and cannot gain additional privilege on the host system by default. Read more about Security in Apptainer.

This project is way older than (rootless) podman.

You should put time into learning this if you are going to be running HPC jobs on clusters, because some HPC clusters support this for jobs and not much else.