An annoying number of Debian installs fail on laptops because the wifi needs firmware and people expect it to work. You can choose to enable non-free after install, but the installer doesn't have that as a direct option.
Prior to this, an alternative installer was available that had the firmware in it... but it was not well-publicized, and was not the default or even available from the same page as the default.
If you install with non-free firmware, it will continue to be installed afterwards.
According to the Debian wiki description of this option[1]: "Where non-free firmware is found to be necessary, the target system will also be configured to use the non-free-firmware component by default in the apt sources.list file."
> The software that is not needed for basic functionality will not be installed permanently, right?
I'm not sure I understand you right. Unless the software is needed, the installer won't load it. And I'm not sure what you mean by "basic functionality" here, but the way I read the Debian pages it's about loading everything needed for the hardware to be fully-operational (or as close to it as is possible).
Also, if there are two driver implementations, as is often the case for GPUs, which will be preferred?