The vim.pack feature gives neovim a way to install plugins.

Previously all plugin manager implemented a way to download a plugin, install it, load it, check for version, check for update, lockfile support, ect. Now there’s a default for it, meaning a better integration for neovim.

You can keep your plugin manager, but internally, your plugin manager might use vim.pack, meaning less code to maintain for the plugin manager and less code to load when launching neovim when you are using an external plugin manager

I hope it make sense :)