Its your call, if you want to use the portal or not use the portal.
Have a quick read through the sections on this page -
https://en.wikipedia.org/wiki/Web_portal#HistoryFor one of my larger sites, i use the portal as a "dynamic start page" for visiting users. it basically summarizes all that is new and happening on the site at this moment.
On another one of my sites, I basically just use the portal as a "welcome page" that has some specifications about the niche product my forum is related to, and a "table of contents" to pages within the site that may be of interest to the user.
Some people have their own "entry/home page" which has a link directly to their forum. So rather than using UBB.threads's built-in portal system, they have their own setup take care of that, and then let UBBT take care of just the forum stuff.
Its totally your call on how you want to setup your site. But the options are there.
