Every job has tools. Beyond those things related to photography, mine are my computer and word-processing program, my dictionaries and reference books, my catalogues and manuals. They all need to work properly and do the job I need them to do. If not, I can't do a good job for my client (you, the reader).
Photography is exactly like that or any other job: you need some tools to do it well, and your tools need to be right for the job you're asking them to do.
All types of photography start out the same way and use the same basic tools, and if you're set up for general photography, the chances are that you will hardly need to add a thing to your collection of photography equipment. By the time people are serious enough to consider going pro, they've usually got a lot of "stuff," as I say.
Portrait photography is a specialty and thus there are some accessories and pieces of equipment specific to it. You wouldn't need a posing stool to make abstract macros of insects, for example, but you will need one to make a head shot of your client. Additionally, many lenses are particularly useful in portrait situations.
In other words, there's some "stuff" you should have when you're making portraits.
I'm not letting any secrets out of the bag when I say that your camera is the most important aspect of being a photographer. Part of me would like to say it's ...