I think there are three primary ways new systems are started, and I think they reveal the reasons, as well.
First, sometimes people see something lacking in the existing styles they are aware of and decide the only remedy is to create their own to solve that problem. Sometimes, it might be for a specific purpose, like someone who decided to create a theoretical MMA-oriented style. Other times, it's to address specific training needs, like how I understand Krav Maga was created. This approach isn't inherently bad. But it's not automatically good, either.
The second way new styles form is almost by accident. A person has been training for years, and developed their own take on their style. As they acquire students, they naturally teach their own version -- and after awhile, it gets recognized as being "their" style. Sometimes, this process is more deliberate than others. I'd put small-circle jujitsu, Ed Parker's American Kenpo, and (according to their history as I understand it), many of the different karate styles developed in this manner.
Finally, there are the styles started to start styles. These are often keyed to egos... And to minimal training in lots of styles. And to lots of unreadable, illegible foreign rank certificates or self-appointed boards and soke-ship councils... Yeah -- these are almost always bad, in my opinion. A few (who generally would argue that they ARE NOT creating a style, just doing their own thing) are good -- but most are about money and power.