This is a long way of addressing your confusion:
Currently Rapid Photo Downloader can be installed two ways: from the Linux distribution’s software repository, or from the installation script I write and maintain.
In an ideal world, it would be better to install the program from the distribution’s software repository. The program would be installed into the standard location programs go. The main problem with that method is that the version is often out of date, or there can be packaging bugs. There can be bugs because Rapid Photo Downloader has a lot of dependencies, which makes it easier for things to go wrong compared to a more simple program.
So I maintain the installation script to be able to deal with complexity of the different Linux distributions. The installation script has two primary tasks: install dependencies using the distribution’s package manager, and then use pip to install a bunch of Python dependencies and the program itself.
Best practice with pip is to install as much as possible on the user’s home directory. Installing Python packages as root with pip is highly recommended against and can easily result in a system that no longer works properly.
If pip did not exist, the installation script would not exist, and users would need to wait 6 or more months to be able to access new versions.
I tried to make a snap package but due to a still outstanding bug in the snap packager, that failed.
AppImage was tried but that effort also failed due to the complexity of the dependencies.
I’ve not tried Flatpak. As far as I know, nobody has attempted to build a Flatpak for Rapid Photo Downloader.
The installation script is a compromise. It’s far from ideal. It means no visibility in the application “stores” the mainstream distributions ship with these days. It takes me time to maintain it. It’s confusing for users who don’t know how to use the command line.
Also please keep in mind that anyone who maintains a FOSS program always needs to make compromises, because we have limited time, and the Linux distribution landscape is so fractured.