Well, the goal of the Software Portal and mintInstall is to provide a (very) easy way to install software (3 clicks required), i'm sure you know that.
There is also an important difference between the portal and synaptic which is : with synaptic, you install packages
whereas with mintinstall, you install applications
Add to this the large amount of packages displayed within synaptic and you can easily understan how a newcomer (and even a not-so-new-comer
) can be overwhelmed.
Thus, the portal is to include software both from the repos and external sources.
We'll try to make it as complete as possible.