Out of all knives I have, they all have some key differences.
If they have similar size blade and shape, they at least have significantly different steel (like carbon vs stainless).
Yet, I see people owning 4-5 same knives, just with handles being in different colors.
Or a guy I know that has VG10 San Mai SRK and has got Fallkniven A1 in that same steel... and then CoS Fallkniven too.
Is it just for collection purposes or addiction or what is the cause?
Long winded reply ahead!
As someone with entirely too many knives in my collection (many of which have a similar blade shape), it's simple. Variety is the spice of life. The days when a mountain man had a single knife on his belt that carried him through his whole life, and which was used to prepare food, skin/quarter an animal, do wood-processing tasks, dig, pry, defend himself from a wild animal, trim materials or plants, and on and on, those days are long gone for a majority of people. The hardest "use" most peoples' knives these days will see are things like opening Amazon packages, opening that clamshell packaged toy for your child, cutting your lunch sandwich in half (and then the apple you packed with it), nipping a thread off a shirt, and so on. And all of those tasks could be handled by virtually any blade shape, with any grind, in almost any steel. That fact isn't really in dispute, and we all know it.
Now, as to your questions: some folks just like variety, even when that variety makes a selection of things identical in any actual use aspect. Me? Hell, here's an example. I have the Spyderco Delica in almost every steel they come in (still missing a few early day oddballs like AUS-6*), and different handle scale colors. A Spyderco Delica, as you might have surmised, in say, ZDP-189 cuts an apple into slices in exactly the same way that one in M4 or even VG-10 does it. I know that, everyone knows that. I just like collecting 'em.
Others will gravitate towards a collction of knives in a steel that they have found works for them, and there's also nothing wrong with that either. Maybe it's the perfect blend of edge holding and sharpenability, or they're an outdoorsman, or Tradesman, and need all day toughness. There's a steel out there for everyone, really. My own Father, I have given the guy thousands of dollars in knives over the years, but know what he likes to carry? A small no-name folder that has a personal inscription on it that another family member gave him from that place in the mall that does graduation/celebration gifts and you can have the gifts personalized. The blade is made from a "steel" is probably 100% comprised of an un-heat-treated sliver stamped from the back bumper of a '73 Super Beetle, but he keeps that thing SHARP, and it does everything he needs.
As for whether it's collecting or an addiction, well...I mean, it's a very fine, blurred line and we ARE on a knife forum which is a safe space for people with that particular affliction....LOL
* Folks, feel free to inbox me if you can help me out!
spot on. I'm firmly in the latter two categories, although I have bought a few duplicates to keep backups on hand in case an especially favored model is lost/broken/discontinued. I imagine most BF members share traits amongst several, if not all categories. 
