1. You just don't know when an idea will strike. Unless you have a photographic memory, you'd want to write down that idea immediately so you may study it at a later time. Ideas often come when you are aware of the sights and sounds around you.
2. It's better to write down experiences as much as possible in real time then try to relive and write about it later. For example a first-time visit to a park or rain forest can produce emotions or feelings that would be difficult to duplicate in writing if recalled by memory when it's long over.
3. You might meet an important or exciting personality whom you'd like to quote verbatim. To write about it later out of memory might cause you to misquote the person, which will render your article false. What's worse is that you might draw the ire of the person misquoted.
4. When you have a chosen subject matter, it helps very much to collect information. An article with lots of information endears you to readers, especially those who go to the Internet for information. Jotting down as much information you can get will also give you fodder for more future articles. Information will also generate those all-imortant ideas.
Of course, it goes without saying, that carrying a notebook will entail carrying that other indispensable tool, a pen (or pencil). Carrying these are unobtrusive, lightweight and can easily be retrieved and used.
They may seem old-fashioned, but they work nonetheless.