Let me take a stab at this.
Lite Software is designed to be a small sub-repository of the most commonly used software new users consider installing after first logging into Linux Lite. Some things, like Restricted Extras, Virtualbox, Clementine, or Steam may be available in the at-large Ubuntu repositories via Install/Remove Software (also known as Synaptic Package Manager). Other things like Dropbox, Teamviewer, and Google Chrome are not.
Either way, the Linux Lite developers have worked hard over the past versions to make these common items as easy to install as putting a check mark next to the desired package within Lite Software. In the process, they spare the new person from having to figure out Synaptic Package Manager, and from having to figure out how to add PPAs, or how to download and run a *.deb package on day one of their Linux Lite experience. Some software overlap will exist between Lite Software and Install/Remove Software, but not a lot.
They complement one another.
