Oracle's not the only vendor with overlapping offerings. IBM and Open Text support multiple Web CMS offerings, BEA sells multiple portals, and so on. In most cases, overlapping offerings result from an acquisition, whereas in Oracle's case, even though the new WebCenter product includes external component, it not acquired from another company.
WebCenter is targeted at organizations that want to build a portal (a website, an intranet, or a traditional enterprise portal) using a J2EE framework as opposed to a Portal framework. It emphasizes so-called "Web 2.0," and some of the key features in current version or planned in the roadmap are discussions, wikis, IM, VoIP, team spaces and mashups -- features you won't find natively in Oracle's Portal product, but can be exposed as portlets. Currently, WebCenter is just a first release whereas the Portal is quite mature. But going forward, Oracle has great plans for WebCenter, including an integration with recently-acquired Stellent.
I suspect that it'll be a long time (if ever) before the company merges its two portal products, and in the meantime, Oracle customers will need to decide between one or both.