You could argue that we had cellphones for ages, why would someone need an iPhone? You could also argue that an android has the same functionality as an iPhone, again why would someone need an iPhone?
The new "category" of software is about how people relate to software, not the software itself. IKEA created a new category of furniture, or it did not?
I think Uber created a new category of service, but we always had taxis and gps; Slack didn't necessarily create new software, but in my opinion it put chat apps on the general public's mind and it kinda became the go-to name, same as AWS or IBM. That's worth something.