There are certain perks offered with paid membership...such as the ability to trade knives, and PMs and such.
More importantly, there are costs involved with running the site incurred by the owner. Paid memberships help defray the costs of operation.
The owner has never required registered members to pay for the privilege of visiting and posting on this site, so the question I suppose you should ask yourself is why should he allow you to do that, and what do you actually contribute to the community?
Frankly none of those perks appeal to me. I like this place but if paid membership was required I would just not come back (or just lurk/read). That’s the owner’s prerogative obviously and if he decided to charge I wouldn’t be upset.