A large number of regular visitors translates into more advertising revenue for them. Other portal sites are created by individual companies to keep you coming back to their products and those of their strategic partners. The default home page for Netscape users is Netscape's home page. This is a site that users can customize to their own needs. Netscape has created strategic partnerships with organizations such as CNN to provide news, The Weather Channel for weather information, CNET for technical news, and so forth.
Some portals are purely topical.
These are still primarily advertiser supported, but have a definite topical focus. Topical sites are not always advertiser supported. They provide links to Linux information with no advertising. The question is whether or not any of these portals work for you. If they don't, you may want to create your own.