The steps give you a clear and helpful structure for your project, but not a rigid one.
With the steps, the 'who does what' - and when - is crystal clear. It's fine for most projects, but you're not locked in. You can alter the sequence of some steps, if you need to, or even skip a step altogether.
In PRIME® all the normal project processes are there, it's just that we have implemented them in an easy step-by-step linear way. That makes your life easier by keeping things really simple and straightforward.
The clearly defined steps also link to advice, checklists and other help so you can get the job done.
Most other project management approaches, including the ISO standard, define many different processes. To manage the different levels of a project these processes then have to be layered or nested. And in many cases the processes overlap too. That makes project process management complicated at best and mind-boggling at worst. And it doesn't always help anyway because focusing on process' takes your eye off the important thing - delivery.