What is the point of PA when all apps are packaged installers?

no, you simply 'install' it directly to your flash. The installer will simply unpack all the parts, creates the needed folders and copy things where they belong so the structure is maintained.

Some of the installers will download the original software from manufacturers repository and copy the parts to the folders which will create the installer.

Just try it and you will see , there is all simple and stright forward. No extra copy steps or so.

A PortableApps.com Installer is very different from a normal "installer". Think of it more like a self-extracting zip file but with additional logic in it to keep things running smoothly. It doesn't require admin rights, makes no changes to the local machine, can be directed to extract wherever you want, etc.

I learned very early on that ZIP files are an awful way to distribute more complicated apps when unzipping one version of Firefox Portable over another wound up breaking it (the app switched DLLs and if the old DLL was present, it would crash). Lots of zipped semi-portable apps will have breaking conditions like this. Or they overwrite your settings if you aren't paying attention (and it's difficult since they all have different places to store your data). That's why we're standardized on a single common open format. Every app works the same. You can always upgrade it just by extracting a new version over the old.

For the full details and advantages, check this list out: https://portableapps.com/about/what_is_a_portable_app#whypaf